Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide, 12.2(58)SE

Configuring IP Unicast Routing

This chapter describes how to configure IP Version 4 (IPv4) unicast routing on the switch. Unless otherwise noted, the term switch refers to a standalone switch and to a switch stack. A switch stack operates and appears as a single router to the rest of the routers in the network.

Basic routing functions, including static routing and the Routing Information Protocol (RIP), are available with both the IP base feature set and the IP services feature set. To use advanced routing features and other routing protocols, you must have the IP services feature set enabled on the standalone switch or on the stack master.

Note If the switch or switch stack is running the IP services feature set, you can also enable IP Version 6 (IPv6) unicast routing and configure interfaces to forward IPv6 traffic in addition to IPv4 traffic. For information about configuring IPv6 on the switch, see Chapter 40 "Configuring IPv6 Unicast Routing."

For more detailed IP unicast configuration information, see the Cisco IOS IP Configuration Guide, Release 12.2 from the Cisco.com page under Documentation > Cisco IOS Software > 12.2 Mainline > Configuration Guides. For complete syntax and usage information for the commands used in this chapter, see these command references from the Cisco.com page under Documentation > Cisco IOS Software > 12.2 Mainline > Command References:

•Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2

•Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2

•Cisco IOS IP Command Reference, Volume 3 of 3: Multicast, Release 12.2

This chapter consists of these sections:

•Understanding IP Routing

•Steps for Configuring Routing

•Configuring IP Addressing

•Enabling IP Unicast Routing

•Configuring RIP

•Configuring OSPF

•Configuring EIGRP

•Configuring BGPConfiguring ISO CLNS Routing

•Configuring Multi-VRF CE

•Configuring Protocol-Independent Features

•Monitoring and Maintaining the IP Network

Note When configuring routing parameters on the switch and to allocate system resources to maximize the number of unicast routes allowed, you can use the sdm prefer routing global configuration command to set the Switch Database Management (SDM) feature to the routing template. For more information on the SDM templates, see Chapter 8 "Configuring SDM Templates," or see the sdm prefer command in the command reference for this release.

Understanding IP Routing

In some network environments, VLANs are associated with individual networks or subnetworks. In an IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the size of the broadcast domain and keeps local traffic local. However, network devices in different VLANs cannot communicate with one another without a Layer 3 device (router) to route traffic between the VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN.

Figure 39-1 shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The router has an interface in each VLAN.

Figure 39-1 Routing Topology Example

When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet addressed to that host. Switch A forwards the packet directly to Host B without sending it to the router.

When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which receives the traffic on the VLAN 10 interface. The router surveys the routing table, finds the correct outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives the packet and forwards it to Host C.

This section contains information on these routing topics:

•Types of Routing

•IP Routing and Switch Stacks

Types of Routing

Routers and Layer 3 switches can route packets in these ways:

•By using default routing

•By using preprogrammed static routes for the traffic

•By dynamically calculating routes by using a routing protocol

Default routing refers to sending traffic with a destination unknown to the router to a default outlet or destination.

Static unicast routing forwards packets from predetermined ports through a single path into and out of a network. Static routing is secure and uses little bandwidth, but it does not automatically respond to changes in the network, such as link failures. Therefore, network changes might result in unreachable destinations. As networks grow, static routing becomes a labor-intensive liability.

Routers use these dynamic routing protocols to dynamically calculate the best route for forwarding traffic:

•Routers that use distance-vector protocols maintain routing tables with distance values of networked resources and periodically pass these tables to their neighbors. Distance-vector protocols use one or a series of metrics for calculating the best routes.

•Routers using link-state protocols maintain a complex database of network topology based on the exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in the network, which speeds up the convergence time or time required to respond to these changes. Link-state protocols respond quickly to topology changes but require greater bandwidth and more resources than distance-vector protocols.

Distance-vector protocols supported by the switch use Routing Information Protocol (RIP), a single-distance metric (cost) that determines the best path, and Border Gateway Protocol (BGP), which adds a path vector mechanism. The switch also supports the Open Shortest Path First (OSPF) link-state protocol and Enhanced IGRP (EIGRP), which adds some link-state routing features to traditional Interior Gateway Routing Protocol (IGRP) to improve efficiency.

Note On a switch or switch stack, the supported protocols are determined by the software running on the switch or stack master. If the switch or stack master is running the IP base feature set, only default routing, static routing and RIP are supported. All other routing protocols require the IP services feature set.

IP Routing and Switch Stacks

A switch stack appears to the network as a single router, regardless of which switch in the stack is connected to a routing peer. For additional information about switch stack operation, see Chapter 7 "Managing Switch Stacks."

The stack master performs these functions:

•It initializes and configures the routing protocols.

•It sends routing protocol messages and updates to other routers.

•It processes routing protocol messages and updates received from peer routers.

•It generates, maintains, and distributes the distributed Cisco Express Forwarding (dCEF) database to all stack members. The routes are programmed on all switches in the stack bases on this database.

•The MAC address of the stack master is used as the router MAC address for the whole stack, and all outside devices use this address to send IP packets to the stack.

•All IP packets that require software forwarding or processing go through the CPU of the stack master.

Stack members perform these functions:

•They act as routing standby switches, ready to take over in case they are elected as the new stack master if the stack master fails.

•They program the routes into hardware. The routes programmed by the stack members are the same that are downloaded by the stack master as part of the dCEF database.

If a stack master fails, the stack detects that the stack master is down and elects one of the stack members to be the new stack master. During this period, except for a momentary interruption, the hardware continues to forward packets with no active protocols.

However, even though the switch stack maintains the hardware identification after a failure, the routing protocols on the router neighbors might flap during the brief interruption before the stack master restarts. Routing protocols such as OSPF and EIGRP need to recognize neighbor transitions. The router uses two levels of nonstop forwarding (NSF) to detect a change-over, to continue forwarding network traffic, and to recover route information from peer devices:

•NSF-aware routers tolerate neighboring router failures. After the neighbor router restarts, an NSF-aware router supplies information about its state and route adjacencies on request.

•NSF-capable routers support NSF. When they detect a stack master change, they rebuild routing information from NSF-aware or NSF-capable neighbors and do not wait for a restart.

The switch stack supports NSF-capable routing for OSPF and EIGRP. For more information, see the "OSPF NSF Capability" section and the "EIGRP NSF Capability" section.

Upon election, the new stack master performs these functions:

•It starts generating, receiving, and processing routing updates.

•It builds routing tables, generates the CEF database, and distributes it to stack members.

•It uses its MAC address as the router MAC address. To notify its network peers of the new MAC address, it periodically (every few seconds for 5 minutes) sends a gratuitous ARP reply with the new router MAC address.

Note If you configure the persistent MAC address feature on the stack and the stack master changes, the stack MAC address does not change for the configured time period. If the previous stack master rejoins the stack as a member switch during that time period, the stack MAC address remains the MAC address of the previous stack master. See the "Enabling Persistent MAC Address" section.

•It attempts to determine the reachability of every proxy ARP entry by sending an ARP request to the proxy ARP IP address and receiving an ARP reply. For each reachable proxy ARP IP address, it generates a gratuitous ARP reply with the new router MAC address. This process is repeated for 5 minutes after a new stack master election.

Note When a stack master is running the IP services feature set, the stack can run all supported protocols, including Open Shortest Path First (OSPF), Enhanced IGRP (EIGRP), and Border Gateway Protocol (BGP). If the stack master fails and the new elected stack master is running the IP base feature set, these protocols no longer run on the stack.

Caution Partitioning on the switch stack into two or more stacks might lead to undesirable behavior in the network.

Steps for Configuring Routing

By default, IP routing is disabled on the switch, and you must enable it before routing can take place. For detailed IP routing configuration information, see the Cisco IOS IP Configuration Guide, Release 12.2 from the Cisco.com page under Documentation > Cisco IOS Software > 12.2 Mainline > Configuration Guides.

In the following procedures, the specified interface must be one of these Layer 3 interfaces:

•A routed port: a physical port configured as a Layer 3 port by using the no switchport interface configuration command.

•A switch virtual interface (SVI): a VLAN interface created by using the interface vlan vlan_id global configuration command and by default a Layer 3 interface.

•An EtherChannel port channel in Layer 3 mode: a port-channel logical interface created by using the interface port-channel port-channel-number global configuration command and binding the Ethernet interface into the channel group. For more information, see the "Configuring Layer 3 EtherChannels" section.

Note The switch does not support tunnel interfaces for unicast routed traffic.

All Layer 3 interfaces on which routing will occur must have IP addresses assigned to them. See the "Assigning IP Addresses to Network Interfaces" section.

A Layer 3 switch can have an IP address assigned to each routed port and SVI. The number of routed ports and SVIs that you can configure is not limited by software. However, the interrelationship between this number and the number and volume of features being implemented might have an impact on CPU utilization because of hardware limitations. To optimize system memory for routing, use the sdm prefer routing global configuration command.

Configuring routing consists of several main procedures:

•To support VLAN interfaces, create and configure VLANs on the switch or switch stack, and assign VLAN membership to Layer 2 interfaces. For more information, see Chapter 13 "Configuring VLANs."

•Configure Layer 3 interfaces.

•Enable IP routing on the switch.

•Assign IP addresses to the Layer 3 interfaces.

•Enable selected routing protocols on the switch.

•Configure routing protocol parameters (optional).

Configuring IP Addressing

A required task for configuring IP routing is to assign IP addresses to Layer 3 network interfaces to enable the interfaces and to allow communication with the hosts on those interfaces that use IP. These sections describe how to configure various IP addressing features. Assigning IP addresses to the interface is required; the other procedures are optional.

•Default Addressing Configuration

•Assigning IP Addresses to Network Interfaces

•Configuring Address Resolution Methods

•Routing Assistance When IP Routing is Disabled

•Configuring Broadcast Packet Handling

•Monitoring and Maintaining IP Addressing

Default Addressing Configuration

Table 39-1 shows the default addressing configuration.

Assigning IP Addresses to Network Interfaces

An IP address identifies a location to which IP packets can be sent. Some IP addresses are reserved for special uses and cannot be used for host, subnet, or network addresses. RFC 1166, "Internet Numbers," contains the official description of IP addresses.

An interface can have one primary IP address. A mask identifies the bits that denote the network number in an IP address. When you use the mask to subnet a network, the mask is referred to as a subnet mask. To receive an assigned network number, contact your Internet service provider.

Beginning in privileged EXEC mode, follow these steps to assign an IP address and a network mask to a Layer 3 interface:

Use of Subnet Zero

We strongly discourage subnetting with a subnet address of zero because of the problems that can arise if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as 255.255.255.0, subnet zero would be written as 131.108.0.0, which is the same as the network address.

You can use the all ones subnet (131.108.255.0) and, even though we discourage this practice, you can enable the subnet zero if you need the entire subnet space for your IP address.

Beginning in privileged EXEC mode, follow these steps to enable subnet zero:

Use the no ip subnet-zero global configuration command to restore the default and to disable the use of subnet zero.

Classless Routing

By default, classless routing is enabled when the switch is configured to route. With classless routing, if a router receives packets for a subnet of a network with no default route, the router forwards the packet to the best supernet route. A supernet is contiguous blocks of Class C address spaces simulating a single, larger address space and is designed to relieve the pressure on the rapidly depleting Class B address space.

In Figure 39-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of discarding the packet, the router forwards it to the best supernet route. If you disable classless routing and a router receives packets destined for a subnet of a network with no network default route, the router discards the packet.

Figure 39-2 IP Classless Routing

In Figure 39-3, the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and 128.20.3.0. If the host sends a packet to 120.20.4.1, because there is no network default route, the router discards the packet.

Figure 39-3 No IP Classless Routing

To prevent the switch from forwarding packets destined for unrecognized subnets to the best supernet route possible, you can disable classless routing behavior.

Beginning in privileged EXEC mode, follow these steps to disable classless routing:

To restore the default so that the switch forwards packets destined for a subnet of a network without a network default route to the best possible supernet route, use the ip classless global configuration command.

Configuring Address Resolution Methods

You can control interface-specific handling of IP by using address resolution. A device using IP can have both a local address or a MAC address, which uniquely defines the device on its local segment or LAN, and a network address, which identifies the network to which the device belongs.

Note In a switch stack, network communication uses a single MAC address and the IP address of the stack.

The local address or the MAC address is known as a data-link address because it is contained in the data-link layer (Layer 2) section of the packet header and is read by data-link (Layer 2) devices. To communicate with an Ethernet device, the software must learn the MAC address of the device. The process of learning the MAC address from an IP address is called address resolution. The process of learning the IP address from the MAC address is called reverse address resolution.

The switch uses these types of address resolution:

•Address Resolution Protocol (ARP) associates IP address with MAC addresses. Using an IP address as input, ARP learns the associated MAC address and then stores the IP address/MAC address association in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests or replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP).

•Proxy ARP helps hosts with no routing tables learn the MAC addresses of hosts on other networks or subnets. If the switch (router) receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to the host through other interfaces, it generates a proxy ARP packet giving its own local data-link address. The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host.

The switch also uses the Reverse Address Resolution Protocol (RARP), which functions the same as ARP does, except that the RARP packets request an IP address instead of a local MAC address. Using RARP requires a RARP server on the same network segment as the router interface. Use the ip rarp-server address interface configuration command to identify the server.

For more information on RARP, see the Cisco IOS Configuration Fundamentals Configuration Guide, Release 12.2 under Documentation > Cisco IOS Software > 12.2 Mainline > Configuration Guides from the Cisco.com page.

You can perform these tasks to configure address resolution:

•Define a Static ARP Cache

•Set ARP Encapsulation

•Enable Proxy ARP

Define a Static ARP Cache

ARP and other address resolution protocols provide dynamic mapping between IP addresses and MAC addresses. Because most hosts support dynamic address resolution, you usually do not need to specify static ARP cache entries. If you must define a static ARP cache entry, you can do so globally, which installs a permanent entry in the ARP cache that the switch uses to translate IP addresses into MAC addresses. You can also specify that the switch respond to ARP requests as if it were the owner of the specified IP address. If you do not want the ARP entry to be permanent, you can specify a timeout period for the ARP entry.

Beginning in privileged EXEC mode, follow these steps to provide static mapping between IP addresses and MAC addresses:

To remove an entry from the ARP cache, use the no arp ip-address hardware-address type global configuration command. To remove all nonstatic entries from the ARP cache, use the clear arp-cache privileged EXEC command.

Set ARP Encapsulation

By default, Ethernet ARP encapsulation (represented by the arpa keyword) is enabled on an IP interface. You can change the encapsulation method to SNAP if required by your network.

Beginning in privileged EXEC mode, follow these steps to specify the ARP encapsulation type:

To disable an encapsulation type, use the no arp arpa or no arp snap interface configuration command.

Enable Proxy ARP

By default, the switch uses proxy ARP to help hosts learn MAC addresses of hosts on other networks or subnets.

Beginning in privileged EXEC mode, follow these steps to enable proxy ARP if it has been disabled:

To disable proxy ARP on the interface, use the no ip proxy-arp interface configuration command.

Routing Assistance When IP Routing is Disabled

These mechanisms allow the switch to learn about routes to other networks when it does not have IP routing enabled:

•Proxy ARP

•Default Gateway

•ICMP Router Discovery Protocol (IRDP)

Proxy ARP

Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no routing information to communicate with hosts on other networks or subnets. The host assumes that all hosts are on the same local Ethernet and that they can use ARP to learn their MAC addresses. If a switch receives an ARP request for a host that is not on the same network as the sender, the switch evaluates whether it has the best route to that host. If it does, it sends an ARP reply packet with its own Ethernet MAC address. The host that sent the request then sends the packet to the switch, which forwards it to the intended host. Proxy ARP treats all networks as if they are local and performs ARP requests for every IP address.

Proxy ARP is enabled by default. To enable it after it has been disabled, see the "Enable Proxy ARP" section. Proxy ARP works as long as other routers support it.

Default Gateway

Another method for locating routes is to define a default router or default gateway. All nonlocal packets are sent to this router, which either routes them appropriately or returns an IP Control Message Protocol (ICMP) redirect message, identifying the local router that the host should use. The switch caches the redirect messages and forwards each packet as efficiently as possible. This method cannot detect when the default router has failed or is unavailable.

Beginning in privileged EXEC mode, follow these steps to define a default gateway (router) when IP routing is disabled:

Use the no ip default-gateway global configuration command to disable this function.

ICMP Router Discovery Protocol (IRDP)

Router discovery allows the switch to dynamically learn about routes to other networks by using ICMP Router Discovery Protocol (IRDP). Hosts use IRDP to locate routers. When operating as a client, the switch generates router discovery packets. When operating as a host, the switch receives router discovery packets. The switch can also receive Routing Information Protocol (RIP) routing updates and use this information to infer locations of routers. The switch does not actually store the routing tables sent by routing devices; it merely keeps track of which systems are sending the data. The advantage of using IRDP is that it allows each router to specify both a priority and the time after which a device is assumed to be down if no more packets are received.

Each device discovered becomes a candidate for the default router, and a new highest-priority router is selected when a higher priority router is discovered, when the current default router is declared down, or when a TCP connection is about to time out because of excessive retransmissions.

The only required task for IRDP routing on an interface is to enable IRDP processing on that interface. When enabled, the default parameters apply. You can change any of these parameters.

Beginning in privileged EXEC mode, follow these steps to enable and configure IRDP on an interface:

If you change the maxadvertinterval value, the holdtime and minadvertinterval values also change. It is important that you first change the maxadvertinterval value, before manually changing either the holdtime or minadvertinterval values.

Use the no ip irdp interface configuration command to disable IRDP routing.

Configuring Broadcast Packet Handling

After configuring an IP interface address, you can enable routing and configure one or more routing protocols, or you can configure the way the switch responds to network broadcasts. A broadcast is a data packet destined for all hosts on a physical network. The switch supports these kinds of broadcasting:

•A directed broadcast packet sent to a specific network or series of networks. A directed broadcast address includes the network or subnet fields.

•A flooded broadcast packet sent to every network.

Note You can also limit broadcast, unicast, and multicast traffic on Layer 2 interfaces by using the storm-control interface configuration command to set traffic suppression levels. For more information, see Chapter 26 "Configuring Port-Based Traffic Control."

Routers provide some protection from broadcast storms by limiting the extent to the local cable. Bridges (including intelligent bridges), because they are Layer 2 devices, forward broadcasts to all network segments, thus propagating broadcast storms. The best solution to the broadcast storm problem is to use a single broadcast address scheme on a network. In most IP implementations, you can set the broadcast address. Many implementations, including the one in the switch, support several addressing schemes for forwarding broadcast messages.

Perform the tasks in these sections to enable these schemes:

•Enabling Directed Broadcast-to-Physical Broadcast Translation

•Forwarding UDP Broadcast Packets and Protocols

•Establishing an IP Broadcast Address

•Flooding IP Broadcasts

Enabling Directed Broadcast-to-Physical Broadcast Translation

By default, IP directed broadcasts are dropped; they are not forwarded. Dropping IP-directed broadcasts makes routers less susceptible to denial-of-service attacks.

You can enable forwarding of IP-directed broadcasts on an interface when the broadcast becomes a physical (MAC-layer) broadcast. Only those protocols configured by using the ip forward-protocol global configuration command are forwarded.

You can specify an access control list (ACL) to control which broadcasts are forwarded. When an ACL is specified, only those IP packets permitted by the ACL can be translated from directed broadcasts to physical broadcasts. For more information on access lists, see Chapter 35 "Configuring Network Security with ACLs."

Beginning in privileged EXEC mode, follow these steps to enable forwarding of IP-directed broadcasts on an interface:

Use the no ip directed-broadcast interface configuration command to disable translation of directed broadcasts to physical broadcasts. Use the no ip forward-protocol global configuration command to remove a protocol or a port.

Forwarding UDP Broadcast Packets and Protocols

User Datagram Protocol (UDP) is an IP host-to-host layer protocol. UDP provides a connectionless session between two end systems and does not acknowledge received datagrams. Network hosts can use UDP broadcasts to find address, configuration, and name information. If such a host is on a network segment that does not include a server, UDP broadcasts might not be forwarded. However, you can configure an interface on a router to forward certain broadcast classes to a helper address. You can use more than one helper address per interface.

You can specify a UDP destination port to control which UDP services are forwarded. You can specify multiple UDP protocols. You can also specify the Network Disk Protocol (NDP), which is used by older diskless Sun workstations and the network security protocol SDNS.

By default, both UDP and NDP forwarding are enabled if a helper address has been defined for an interface. The description for the ip forward-protocol interface configuration command in the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2 lists the ports that are forwarded by default if you do not specify any UDP ports.

If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry DHCP information.

Beginning in privileged EXEC mode, follow these steps to enable forwarding of UDP broadcast packets on an interface and to specify the destination address:

Use the no ip helper-address interface configuration command to disable the forwarding of broadcast packets to specific addresses. Use the no ip forward-protocol global configuration command to remove a protocol or port.

Establishing an IP Broadcast Address

The IP broadcast address (and the default) is an address consisting of all ones (255.255.255.255). However, the switch can be configured to generate any form of IP broadcast address.

Beginning in privileged EXEC mode, follow these steps to set the IP broadcast address on an interface:

To restore the default IP broadcast address, use the no ip broadcast-address interface configuration command.

Flooding IP Broadcasts

You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion by using the database created by the bridging STP. Using this feature also prevents loops. To support this capability, you must configure bridging on each interface that is to participate in the flooding. If bridging is not configured on an interface, it still can receive broadcasts. However, the interface never forwards broadcasts it receives, and the router never uses that interface to send broadcasts received on a different interface.

Packets that are forwarded to a single network address using the IP helper-address mechanism can be flooded. Only one copy of the packet is sent on each network segment.

To be considered for flooding, packets must meet these criteria. (Note that these are the same conditions to be met for packet forwarding when using IP helper addresses.)

•The packet must be a MAC-level broadcast.

•The packet must be an IP-level broadcast.

•The packet must be a TFTP, DNS, Time, NetBIOS, Network Disk, or BOOTP packet, or a UDP specified by the ip forward-protocol udp global configuration command.

•The time-to-live (TTL) value of the packet must be at least 2.

A flooded UDP datagram is given the destination address specified with the ip broadcast-address interface configuration command on the output interface. The destination address can be set to any address. Thus, the destination address might change as the datagram propagates through the network. The source address never changed. The TTL value decrements.

When a flooded UDP datagram is sent on an interface (and the destination address is possibly changed), the datagram is processed by the normal IP output routines and is, therefore, subject to ACLs, if they are present on the output interface.

Beginning in privileged EXEC mode, follow these steps to use the bridging spanning-tree database to flood UDP datagrams:

Use the no ip forward-protocol spanning-tree global configuration command to disable the flooding of IP broadcasts.

In the switch, the majority of packets are forwarded in hardware; most packets do not go through the switch CPU. For those packets that do go to the CPU, you can speed up spanning tree-based UDP flooding by a factor of about four to five times by using turbo-flooding. This feature is supported over Ethernet interfaces configured for ARP encapsulation.

Beginning in privileged EXEC mode, follow these steps to increase spanning-tree-based flooding:

To disable this feature, use the no ip forward-protocol turbo-flood global configuration command.

Monitoring and Maintaining IP Addressing

When the contents of a particular cache, table, or database have become or are suspected to be invalid, you can remove all its contents by using the clear privileged EXEC commands. Table 39-2 lists the commands for clearing contents.

You can display specific statistics, such as the contents of IP routing tables, caches, and databases; the reachability of nodes; and the routing path that packets are taking through the network. Table 39-3 lists the privileged EXEC commands for displaying IP statistics.

Enabling IP Unicast Routing

By default, the switch is in Layer 2 switching mode, and IP routing is disabled. To use the Layer 3 capabilities of the switch, you must enable IP routing.

Beginning in privileged EXEC mode, follow these steps to enable IP routing:

Use the no ip routing global configuration command to disable routing.

This example shows how to enable IP routing by using RIP as the routing protocol:

Switch# configure terminal 

Enter configuration commands, one per line.  End with CNTL/Z.

Switch(config)# ip routing 

Switch(config)# router rip 

Switch(config-router)# network 10.0.0.0 

Switch(config-router)# end 

You can now set parameters for the selected routing protocols as described in these sections:

•Configuring RIP

•Configuring OSPF

•Configuring EIGRP

•Configuring BGP

•Configuring Unicast Reverse Path Forwarding

•Configuring Protocol-Independent Features (optional)

Configuring RIP

The Routing Information Protocol (RIP) is an interior gateway protocol (IGP) for use in small, homogeneous networks. It is a distance-vector routing protocol that uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. You can find detailed information about RIP in IP Routing Fundamentals, published by Cisco Press.

Note RIP is the only routing protocol supported by the IP base feature set; other routing protocols require the switch or the stack master to be running the IP services feature set.

Using RIP, the switch sends routing information updates (advertisements) every 30 seconds. If a router does not receive an update from another router for 180 seconds or more, it marks the routes served by that router as unusable. If there is still no update after 240 seconds, the router removes all routing table entries for that router.

RIP uses hop counts to rate the value of different routes. The hop count is the number of routers that can be traversed in a route. The hop count range is 0 to 15. A directly connected network has a hop count of zero; a network with a hop count of 16 is unreachable. The small range makes RIP unsuitable for large networks.

If the router has a default network path, RIP advertises a route that links the router to the pseudonetwork 0.0.0.0. The 0.0.0.0 network does not exist: It is treated by RIP as a network to implement the default routing feature. The switch advertises the default network if a default was learned by RIP or if the router has a gateway of last resort and RIP is configured with a default metric. RIP sends updates to the interfaces in specified networks. If an interface's network is not specified, it is not advertised in any RIP update.

These sections contain this configuration information:

•Default RIP Configuration

•Configuring Basic RIP Parameters

•Configuring RIP Authentication

•Configuring Summary Addresses and Split Horizon

Default RIP Configuration

Table 39-4 shows the default RIP configuration.

Configuring Basic RIP Parameters

To configure RIP, you enable RIP routing for a network and optionally configure other parameters. RIP configuration commands are ignored on the switch until you configure the network number.

Beginning in privileged EXEC mode, follow these steps to enable and configure RIP:

To turn off the RIP routing process, use the no router rip global configuration command.

To display the parameters and current state of the active routing protocol process, use the show ip protocols privileged EXEC command. To display summary address entries in the RIP database, use the show ip rip database privileged EXEC command.

Configuring RIP Authentication

RIP Version 1 does not support authentication. If you are sending and receiving RIP Version 2 packets, you can enable RIP authentication on an interface. The key chain specifies the set of keys that can be used on the interface. If a key chain is not configured, no authentication is performed, not even the default. Therefore, you must also perform the tasks in the "Managing Authentication Keys" section.

The switch supports these modes of authentication on interfaces for which RIP authentication is enabled: plain text and MD5. The default is plain text.

Beginning in privileged EXEC mode, follow these steps to configure RIP authentication on an interface:

To restore clear text authentication, use the no ip rip authentication mode interface configuration command. To prevent authentication, use the no ip rip authentication key-chain interface configuration command.

Configuring Summary Addresses and Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router on any interface from which that information originated. This feature usually optimizes communication among multiple routers, especially when links are broken.

Note In general, we do not recommend disabling split horizon unless you are certain that your application requires it to properly advertise routes.

If you want to configure an interface running RIP to advertise a summarized local IP address pool on a network access server for dial-up clients, use the ip summary-address rip interface configuration command.

Beginning in privileged EXEC mode, follow these steps to set an interface to advertise a summarized local IP address and to disable split horizon on the interface:

To disable IP summarization, use the no ip summary-address rip router configuration command.

In this example, the major net is 10.0.0.0. The summary address of 10.2.0.0 overrides the autosummary address of 10.0.0.0 so that 10.2.0.0 is advertised from interface Gigabit Ethernet port 2, and 10.0.0.0 is not advertised. In the example, if the interface is still in Layer 2 mode (the default), you must enter the no switchport interface configuration command before entering the ip address interface configuration command.

Note If split horizon is enabled, neither autosummary nor interface summary addresses (those configured with the ip summary-address rip router configuration command) are advertised.

Switch(config)# router rip

Switch(config-router)# interface gigabitethernet1/0/2

Switch(config-if)# ip address 10.1.5.1 255.255.255.0

Switch(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0

Switch(config-if)# no ip split-horizon

Switch(config-if)# exit

Switch(config)# router rip

Switch(config-router)# network 10.0.0.0

Switch(config-router)# neighbor 2.2.2.2 peer-group mygroup

Switch(config-router)# end

Configuring Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router on any interface from which that information originated. This feature can optimize communication among multiple routers, especially when links are broken.

Note In general, we do not recommend disabling split horizon unless you are certain that your application requires it to properly advertise routes.

Beginning in privileged EXEC mode, follow these steps to disable split horizon on the interface:

To enable the split horizon mechanism, use the ip split-horizon interface configuration command.

Configuring OSPF

This section briefly describes how to configure Open Shortest Path First (OSPF). For a complete description of the OSPF commands, see the "OSPF Commands" chapter of the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

Note OSPF classifies different media into broadcast, nonbroadcast, and point-to-point networks. The switch supports broadcast (Ethernet, Token Ring, and FDDI) and point-to-point networks (Ethernet interfaces configured as point-to-point links).

OSPF is an Interior Gateway Protocol (IGP) designed expressly for IP networks, supporting IP subnetting and tagging of externally derived routing information. OSPF also allows packet authentication and uses IP multicast when sending and receiving packets.

The Cisco implementation conforms to the OSPF Version 2 specifications with these key features:

•Definition of stub areas is supported.

•Routes learned through any IP routing protocol can be redistributed into another IP routing protocol. At the intradomain level, OSPF can import routes learned through EIGRP and RIP. OSPF routes can also be exported into RIP.

•Plain text and MD5 authentication among neighboring routers within an area is supported.

•Configurable routing interface parameters include interface output cost, retransmission interval, interface transmit delay, router priority, router dead and hello intervals, and authentication key.

•Virtual links are supported.

•Not-so-stubby-areas (NSSAs) per RFC 1587are supported.

OSPF typically requires coordination among many internal routers, area border routers (ABRs) connected to multiple areas, and autonomous system boundary routers (ASBRs). The minimum configuration would use all default parameter values, no authentication, and interfaces assigned to areas. If you customize your environment, you must ensure coordinated configuration of all routers.

These sections contain this configuration information:

•Default OSPF Configuration

•Configuring Basic OSPF Parameters

•Configuring OSPF Interfaces

•Configuring OSPF Area Parameters

•Configuring Other OSPF Parameters

•Changing LSA Group Pacing

•Configuring a Loopback Interface

•Monitoring OSPF

Default OSPF Configuration

Table 39-5 shows the default OSPF configuration.

OSPF for Routed Access

With Cisco IOS Release 12.2(55)SE, the IP Base image supports OSPF for routed access. The IP services image is required if you need multiple OSPFv2 and OSPFv3 instances without route restrictions. Additionally, the IP services image is required to enable the multi-VRF-CE feature.

OSPF for Routed Access is specifically designed so that you can extend Layer 3 routing capabilities to the wiring closet.

Note OSPF for Routed Access supports only one OSPFv2 and one OSPFv3 instance with a combined total of 200 dynamically learned routes. The IP Base image provides OSPF for routed access.

However, these restrictions are not enforced in this release.

With the typical topology (hub and spoke) in a campus environment, where the wiring closets (spokes) are connected to the distribution switch (hub) that forwards all nonlocal traffic to the distribution layer, the wiring closet switch need not hold a complete routing table. A best practice design, where the distribution switch sends a default route to the wiring closet switch to reach interarea and external routes (OSPF stub or totally stub area configuration) should be used when OSPF for Routed Access is used in the wiring closet.

For more details, perform a Google search on "High Availability Campus Network Design" for the routed access layer using EIGRP or OSPF.

OSPF Nonstop Forwarding

The switch or switch stack supports two levels of nonstop forwarding (NSF):

•OSPF NSF Awareness

•OSPF NSF Capability

OSPF NSF Awareness

The IP-services feature set supports OSPF NSF awareness for IPv4. When the neighboring router is NSF-capable, the Layer 3 switch continues to forward packets from the neighboring router during the interval between the primary route processor in a router failure and the take-over of the backup route processor, or while the primary route processor is manually reloaded for a nondisruptive software upgrade.

This feature cannot be disabled. For more information on this feature, see the "OSPF Nonstop Forwarding (NSF) Awareness" section of the Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4.

OSPF NSF Capability

Beginning with Cisco IOS Release 12.2(58)SE, the switch supports the OSPFv2 NSF IETF format in addition to the the OSPFv2 NSF Cisco format that is supported in earlier releases. For information about this feature, see NSF—OSPF (RFC 3623 OSPF Graceful Restart).

The IP-services feature set also supports OSPF NSF-capable routing for IPv4 for better convergence and lower traffic loss following a stack-master change. When a stack-master change occurs in an OSPF NSF-capable stack, the new stack master must do two things to resynchronize its link-state database with its OSFP neighbors:

•Release the available OSPF neighbors on the network without resetting the neighbor relationship.

•Re-acquire the contents of the link-state database for the network.

After a stack-master change, the new master sends an OSPF NSF signal to neighboring NSF-aware devices. A device recognizes this signal to mean that it should not reset the neighbor relationship with the stack. As the NSF-capable stack master receives signals from other routes on the network, it begins to rebuild its neighbor list.

When the neighbor relationships are reestablished, the NSF-capable stack master resynchronizes its database with its NSF-aware neighbors, and routing information is exchanged between the OSPF neighbors. The new stack master uses this routing information to remove stale routes, to update the routing information database (RIB), and to update the forwarding information base (FIB) with the new information. The OSPF protocols then fully converge.

Note OSPF NSF requires that all neighbor networking devices be NSF-aware. If an NSF-capable router discovers non-NSF-aware neighbors on a network segment, it disables NSF capabilities for that segment. Other network segments where all devices are NSF-aware or NSF-capable continue to provide NSF capabilities.

Use the nsf OSPF routing configuration command to enable OSPF NSF routing. Use the show ip ospf privileged EXEC command to verify that it is enabled.

Note NSF is not supported on interfaces configured for Hot Standby Router Protocol (HSRP).

Configuring Basic OSPF Parameters

Enabling OSPF requires that you create an OSPF routing process, specify the range of IP addresses to be associated with the routing process, and assign area IDs to be associated with that range. Beginning in Cisco IOS Release 12.2(58)SE, you can configure either the Cisco OSPFv2 NSF format or the IETF OSPFv2 NSF format.

Beginning in privileged EXEC mode, follow these steps to enable OSPF:

To end an OSPF routing process, use the no router ospf process-id global configuration command.

This example shows how to configure an OSPF routing process and assign it a process ID of 109:

Switch(config)# router ospf 109

Switch(config-router)# network 131.108.0.0 255.255.255.0 area 24

Configuring OSPF Interfaces

You can use the ip ospf interface configuration commands to modify interface-specific OSPF parameters. You are not required to modify any of these parameters, but some interface parameters (hello interval, dead interval, and authentication key) must be consistent across all routers in an attached network. If you modify these parameters, be sure all routers in the network have compatible values.

Beginning in privileged EXEC mode, follow these steps to modify OSPF interface parameters:

Use the no form of these commands to remove the configured parameter value or to return to the default value.

Configuring OSPF Area Parameters

You can optionally configure several OSPF area parameters. These parameters include authentication for password-based protection against unauthorized access to an area, stub areas, and not-so-stubby-areas (NSSAs). Stub areas are areas into which information on external routes is not sent. Instead, the area border router (ABR) generates a default external route into the stub area for destinations outside the autonomous system. An NSSA does not flood all LSAs from the core into the area, but can import autonomous-system external routes within the area by redistribution.

Route summarization consolidates advertised addresses into a single summary route to be advertised by other areas. If network numbers are contiguous, you can use the area range router configuration command to configure the ABR to advertise a summary route that covers all networks in the range.

Note The OSPF area router configuration commands are all optional.

Beginning in privileged EXEC mode, follow these steps to configure area parameters:

Use the no form of these commands to remove the configured parameter value or to return to the default value.

Configuring Other OSPF Parameters

You can optionally configure other OSPF parameters from router configuration mode.

•Route summarization: When redistributing routes from other protocols, as described in the "Using Route Maps to Redistribute Routing Information" section, each route is advertised individually in an external LSA. To help decrease the size of the OSPF link state database, you can use the summary-address router configuration command to advertise a single router for all the redistributed routes included in a specified network address and mask.

•Virtual links: In OSPF, all areas must be connected to a backbone area. You can establish a virtual link in case of a backbone-continuity break by configuring two ABRs as endpoints of a virtual link. Configuration information includes the identity of the other virtual endpoint (the other ABR) and the nonbackbone link that the two routers have in common (the transit area). Virtual links cannot be configured through a stub area.

•Default route: When you specifically configure redistribution of routes into an OSPF routing domain, the route automatically becomes an autonomous system boundary router (ASBR). You can force the ASBR to generate a default route into the OSPF routing domain.

•Domain name server (DNS) names for use in all OSPF show privileged EXEC command displays makes it easier to identify a router than displaying it by router ID or neighbor ID.

•Default metrics: OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. The metric is calculated as ref-bw divided by bandwidth, where ref is 10 by default, and bandwidth (bw) is specified by the bandwidth interface configuration command. For multiple links with high bandwidth, you can specify a larger number to differentiate the cost on those links.

•Administrative distance is a rating of the trustworthiness of a routing information source, an integer between 0 and 255, with a higher value meaning a lower trust rating. An administrative distance of 255 means the routing information source cannot be trusted at all and should be ignored. OSPF uses three different administrative distances: routes within an area (intra-area), routes to another area (interarea), and routes from another routing domain learned through redistribution (external). You can change any of the distance values.

•Passive interfaces: Because interfaces between two devices on an Ethernet represent only one network segment, to prevent OSPF from sending hello packets for the sending interface, you must configure the sending device to be a passive interface. Both devices can identify each other through the hello packet for the receiving interface.

•Route calculation timers: You can configure the delay time between when OSPF receives a topology change and when it starts the shortest path first (SPF) calculation and the hold time between two SPF calculations.

•Log neighbor changes: You can configure the router to send a syslog message when an OSPF neighbor state changes, providing a high-level view of changes in the router.

Beginning in privileged EXEC mode, follow these steps to configure these OSPF parameters:

Changing LSA Group Pacing

The OSPF LSA group pacing feature allows the router to group OSPF LSAs and pace the refreshing, check-summing, and aging functions for more efficient router use. This feature is enabled by default with a 4-minute default pacing interval, and you will not usually need to modify this parameter. The optimum group pacing interval is inversely proportional to the number of LSAs that the router is refreshing, check-summing, and aging. For example, if you have approximately 10,000 LSAs in the database, decreasing the pacing interval would benefit you. If you have a very small database (40 to 100 LSAs), increasing the pacing interval to 10 to 20 minutes might benefit you slightly.

Beginning in privileged EXEC mode, follow these steps to configure OSPF LSA pacing:

To return to the default value, use the no timers lsa-group-pacing router configuration command.

Configuring a Loopback Interface

OSPF uses the highest IP address configured on the interfaces as its router ID. If this interface is down or removed, the OSPF process must recalculate a new router ID and resend all its routing information through its interfaces. If a loopback interface is configured with an IP address, OSPF uses this IP address as its router ID, even if other interfaces have higher IP addresses. Loopback interfaces never fail, which provides greater stability. OSPF automatically prefers a loopback interface over other interfaces, and it chooses the highest IP address among all loopback interfaces.

Beginning in privileged EXEC mode, follow these steps to configure a loopback interface:

Use the no interface loopback 0 global configuration command to disable the loopback interface.

Monitoring OSPF

You can display specific statistics such as the contents of IP routing tables, caches, and databases.

Table 39-6 lists some of the privileged EXEC commands for displaying statistics. For more show ip ospf database privileged EXEC command options and for explanations of fields in the resulting display, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

Configuring EIGRP

Note If the switch is running the IP base image, you can configure complete EIGRP routing. However, the configuration is not implemented because the IP base image supports only EIGRP stub routing.

After you have entered the eigrp stub router configuration command, only the eigrp stub connected summary command takes effect. Although the CLI help might show the receive-only and static keywords, and you can enter these keywords, the switch running the IP base image always behaves as if the connected and summary keywords were configured.

Enhanced IGRP (EIGRP) is a Cisco-proprietary enhanced version of the IGRP. EIGRP uses the same distance vector algorithm and distance information as IGRP; however, the convergence properties and the operating efficiency of EIGRP are significantly improved.

The convergence technology employs an algorithm referred to as the diffusing update algorithm (DUAL), which guarantees loop-free operation at every instant throughout a route computation. All devices involved in a topology change can synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations.

IP EIGRP provides increased network width. With RIP, the largest possible width of your network is 15 hops. Because the EIGRP metric is large enough to support thousands of hops, the only barrier to network expansion is the transport-layer hop counter. EIGRP increments the transport control field only when an IP packet has traversed 15 routers and the next hop to the destination was learned through EIGRP. When a RIP route is used as the next hop to the destination, the transport control field increments as usual.

EIGRP offers these features:

•Fast convergence.

•Incremental updates when the state of a destination changes rather than sending the entire contents of the routing table, minimizing the bandwidth required for EIGRP packets.

•Less CPU usage because full update packets need not be processed each time they are received.

•Protocol-independent neighbor discovery mechanism to learn about neighboring routers.

•Variable-length subnet masks (VLSMs).

•Arbitrary route summarization.

•Scalable for large networks.

EIGRP has these four basic components:

•Neighbor discovery and recovery is the process that routers use to dynamically learn of other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. Neighbor discovery and recovery is achieved by periodically sending small hello packets. As long as hello packets are received, the Cisco IOS software learns that a neighbor is alive and functioning. When this status is determined, the neighboring routers can exchange routing information.

•The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP packets must be sent reliably, and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multi-access network with multicast capabilities (such as Ethernet), it is not necessary to send hellos reliably to all neighbors individually. Therefore, EIGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets (such as updates) require acknowledgment, which is shown in the packet. The reliable transport has a provision to send multicast packets quickly when there are unacknowledged packets pending. Doing so helps ensure that convergence time remains low in the presence of varying speed links.

•The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses the distance information (known as a metric) to select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors, but there are neighbors advertising the destination, a recomputation occurs. This is the process whereby a new successor is determined. The amount of time it takes to recompute the route affects the convergence time. Recomputation is processor-intensive; it is advantageous to avoid recomputation if it is not necessary. When a topology change occurs, DUAL tests for feasible successors. If there are feasible successors, it uses any it finds to avoid unnecessary recomputation.

•The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the IP EIGRP module, which is responsible for sending and receiving EIGRP packets that are encapsulated in IP. It is also responsible for parsing EIGRP packets and informing DUAL of the new information received. EIGRP uses DUAL for routing decisions, but the results are stored in the IP routing table. EIGRP is also responsible for redistributing routes learned by other IP routing protocols.

These sections contain this configuration information:

•Default EIGRP Configuration

•Configuring Basic EIGRP Parameters

•Configuring EIGRP Interfaces

•Configuring EIGRP Route Authentication

•EIGRP Stub Routing

•Monitoring and Maintaining EIGRP

Note To enable EIGRP, the switch or stack master must be running the IP services feature set.

Default EIGRP Configuration

Table 39-7 shows the default EIGRP configuration.

To create an EIGRP routing process, you must enable EIGRP and associate networks. EIGRP sends updates to the interfaces in the specified networks. If you do not specify an interface network, it is not advertised in any EIGRP update.

If you have routers on your network that are configured for IGRP, and you want to change to EIGRP, you must designate transition routers configured with both IGRP and EIGRP. In these cases, perform Steps 1 through 3 in the next section and also see the "Configuring Split Horizon" section. You must use the same autonomous-system number for routes so that they are automatically redistributed.

EIGRP Nonstop Forwarding

The switch stack supports two levels of EIGRP nonstop forwarding:

•EIGRP NSF Awareness

•EIGRP NSF Capability

EIGRP NSF Awareness

The IP-services feature set supports EIGRP NSF awareness for IPv4. When the neighboring router is NSF-capable, the Layer 3 switch continues to forward packets from the neighboring router during the interval between the primary route processor in a router failure and the backup RP take-over, or while you manually reload the primary route processor for a nondisruptive software upgrade.

This feature cannot be disabled. For more information on this feature, see the "EIGRP Nonstop Forwarding (NSF) Awareness" section of the Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4.

EIGRP NSF Capability

Beginning with Cisco IOS Release 12.2(58)SE, the switch supports EIGRP Cisco NSF routing to speed up convergence and eliminate traffic loss following a stack master change. For details about this NSF capability, see the "Configuring Nonstop Forwarding" chapter in the High Availability Configuration Guide, Cisco IOS XE Release 3S at:
http://www.cisco.com/en/US/docs/ios/ios_xe/ha/configuration/guide/ha-nonstp_fwdg_xe.html#wp1085061

The IP-services feature set also supports EIGRP NSF-capable routing for IPv4 for better convergence and lower traffic loss following a stack-master change. When an EIGRP NSF-capable stack master restarts or a new stack master starts and NSF restarts, the switch has no neighbors, and the topology table is empty. The switch must bring up the interfaces, re-acquire neighbors, and rebuild the topology and routing tables without interrupting the traffic directed to the switch stack. EIGRP peer routers maintain the routes learned from the new stack master and continue to forward traffic through the NSF restart process.

To prevent an adjacency reset by the neighbors, the new stack master uses a new restart bit in the EIGRP packet header. When the neighbor receives it, it synchronizes the stack in its peer list and maintains the adjacency with the stack. The neighbor then sends its topology table to the stack master with the restart bit set to show that it is NSF-aware and is aiding the new stack master.

If at least one of the stack peer neighbors is NSF-aware, the stack master receives updates and rebuilds its database. Each NSF-aware neighbor sends an end-of-table (EOT) marker in the last update packet to mark the end of the table content. The stack master recognizes the convergence when it receives the EOT marker and begins sending updates. When the stack master has received all EOT markers from its neighbors or when the NSF converge timer expires, EIGRP notifies the routing information database (RIB) of convergence and floods its topology table to all NSF-aware peers.

Note NSF is not supported on interfaces configured for Hot Standby Router Protocol (HSRP).

Use the nsf EIGRP routing configuration command to enable EIGRP NSF routing. Use the show ip protocols privileged EXEC command to verify that NSF is enabled. See the command reference for this release for information about the nsf command.

Configuring Basic EIGRP Parameters

Beginning in privileged EXEC mode, follow these steps to configure EIGRP. Configuring the routing process is required; other steps are optional:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router eigrp autonomous-system	Enable an EIGRP routing process, and enter router configuration mode. The autonomous-system number identifies the routes to other EIGRP routers and tags routing information.
Step 3	nsf	(Optional) Enable EIGRP NSF. Enter this command on the stack master and on all of its peers.
Step 4	network network-number	Associate networks with an EIGRP routing process. EIGRP sends updates to the interfaces in the specified networks.
Step 5	eigrp log-neighbor-changes	(Optional) Enable logging of EIGRP neighbor changes to monitor routing system stability.
Step 6	metric weights tos k1 k2 k3 k4 k5	(Optional) Adjust the EIGRP metric. Although the defaults have been carefully set to provide excellent operation in most networks, you can adjust them. Caution Setting metrics is complex, and we do not recommend doing without guidance from an experienced network designer.
Step 7	offset list [access-list number | name] {in | out} offset [type number]	(Optional) Apply an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through EIGRP. You can limit the offset list with an access list or an interface.
Step 8	auto-summary	(Optional) Enable automatic summarization of subnet routes into network-level routes.
Step 9	ip summary-address eigrp autonomous-system-number address mask	(Optional) Configure a summary aggregate.
Step 10	end	Return to privileged EXEC mode.
Step 11	show ip protocols	Verify your entries.
Step 12	show ip protocols	Verify your entries. For NSF awareness, the output shows: *** IP Routing is NSF aware *** EIGRP NSF enabled
Step 13	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Configuring EIGRP Interfaces

Other optional EIGRP parameters can be configured on an interface basis.

Beginning in privileged EXEC mode, follow these steps to configure EIGRP interfaces:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	ip bandwidth-percent eigrp percent	(Optional) Configure the percentage of bandwidth that can be used by EIGRP on an interface. The default is 50 percent.
Step 4	ip summary-address eigrp autonomous-system-number address mask	(Optional) Configure a summary aggregate address for a specified interface (not usually necessary if auto-summary is enabled).
Step 5	ip hello-interval eigrp autonomous-system-number seconds	(Optional) Change the hello-time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 60 seconds for low-speed NBMA networks and 5 seconds for all other networks.
Step 6	ip hold-time eigrp autonomous-system-number seconds	(Optional) Change the hold-time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 180 seconds for low-speed NBMA networks and 15 seconds for all other networks. Caution Do not adjust the hold time without consulting Cisco technical support.
Step 7	no ip split-horizon eigrp autonomous-system-number	(Optional) Disable split horizon to allow route information to be advertised by a router from any interface that originated that information.
Step 8	end	Return to privileged EXEC mode.
Step 9	show ip eigrp interface	Display which interfaces EIGRP is active on and information about EIGRP relating to those interfaces.
Step 10	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no forms of these commands to disable the feature or to return the setting to the default value.

Configuring EIGRP Route Authentication

EIGRP route authentication provides MD5 authentication of routing updates from the EIGRP routing protocol to prevent the introduction of unauthorized or false routing messages from unapproved sources.

Beginning in privileged EXEC mode, follow these steps to enable authentication:

Use the no forms of these commands to disable the feature or to return the setting to the default value.

EIGRP Stub Routing

Note The IP base feature set contains EIGRP stub routing capability, which only advertises connected or summary routes from the routing tables to other switches in the network. The switch uses EIGRP stub routing at the access layer to eliminate the need for other types of routing advertisements. For enhanced capability and complete EIGRP routing, the switch must be running the IP services feature set.
On a switch running the IP base feature set, if you try to configure multi-VRF-CE and EIGRP stub routing at the same time, the configuration is not allowed.

In a network using EIGRP stub routing, the only allowable route for IP traffic to the user is through a switch that is configured with EIGRP stub routing. The switch sends the routed traffic to interfaces that are configured as user interfaces or are connected to other devices.

When using EIGRP stub routing, you need to configure the distribution and remote routers to use EIGRP and to configure only the switch as a stub. Only specified routes are propagated from the switch. The switch responds to all queries for summaries, connected routes, and routing updates.

Any neighbor that receives a packet informing it of the stub status does not query the stub router for any routes, and a router that has a stub peer does not query that peer. The stub router depends on the distribution router to send the proper updates to all peers.

In Figure 39-4, Switch B is configured as an EIGRP stub router. Switches A and C are connected to the rest of the WAN. Switch B advertises connected, static, redistribution, and summary routes to Switches A and C. Switch B does not advertise any routes learned from switch A (and the reverse).

Figure 39-4 EIGRP Stub Router Configuration

For more information about EIGRP stub routing, see "Configuring EIGRP Stub Routing" section of the Cisco IOS IP Configuration Guide, Volume 2 of 3: Routing Protocols, Release 12.2.

Monitoring and Maintaining EIGRP

You can delete neighbors from the neighbor table. You can also display various EIGRP routing statistics. Table 39-8 lists the privileged EXEC commands for deleting neighbors and displaying statistics. For explanations of fields in the resulting display, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

Configuring BGP

The Border Gateway Protocol (BGP) is an exterior gateway protocol for an interdomain routing system that guarantees the loop-free exchange of routing information between autonomous systems. Autonomous systems are made up of routers operating under the same administration and run Interior Gateway Protocols (IGPs), such as RIP or OSPF, within their boundaries and interconnecting by using an Exterior Gateway Protocol (EGP). BGP Version 4 is the standard EGP for interdomain routing in the Internet. You can find detailed information about BGP in Internet Routing Architectures, published by Cisco Press, and in the "Configuring BGP" chapter in the Cisco IP and IP Routing Configuration Guide.

For details about BGP commands and keywords, see the "IP Routing Protocols" part of the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2. For a list of BGP commands that are visible but not supported by the switch, see "Unsupported Commands in Cisco IOS Release 12.2(58)SE."

Routers belonging to the same autonomous system and exchanging BGP updates run internal BGP (IBGP). Routers belonging to different autonomous systems and exchanging BGP updates run external BGP (EBGP). Most configuration commands are the same for configuring EBGP and IBGP. The difference is that the routing updates are exchanged either between autonomous systems (EBGP) or within an autonomous system (IBGP). Figure 39-5 shows a network that is running both EBGP and IBGP.

Figure 39-5 EBGP, IBGP, and Multiple Autonomous Systems

Before exchanging information with an external autonomous system, BGP ensures that networks in the autonomous system can be reached by defining internal BGP peering among routers and by redistributing BGP routing information to IGPs that run in the autonomous system, such as IGRP and OSPF.

Routers that run a BGP routing process are often referred to as BGP speakers. BGP uses the Transmission Control Protocol (TCP) as its transport protocol (specifically port 179). Two BGP speakers that have a TCP connection to each other are known as peers or neighbors. In Figure 39-5, Routers A and B are BGP peers, as are Routers B and C and Routers C and D. The routing information is a series of autonomous-system numbers that describe the full path to the destination network. BGP uses this information to construct a loop-free map of autonomous systems.

The network has these characteristics:

•Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP peers are directly connected and that the IBGP peers are not. As long as an IGP allows the two neighbors to reach one another, IBGP peers do not have to be directly connected.

•All BGP speakers within an autonomous system must establish a peer relationship. That is, the BGP speakers within an autonomous system must have a logical full mesh. However, BGP4 provides techniques to reduce the requirement for a logical full mesh: confederations and route reflectors.

•Autonomous system AS 200 is a transit autonomous system for AS 100 and AS 300—that is, AS 200 transfers packets between AS 100 and AS 300.

BGP peers first exchange their full BGP routing tables and then send only incremental updates. BGP peers also exchange keepalive messages (to ensure that the connection is up) and notification messages (in response to errors or special conditions).

In BGP, each route consists of a network number, a list of autonomous systems that information has passed through (the autonomous system path), and a list of other path attributes. The primary function of a BGP system is to exchange network reachability information, including information about the list of autonomous-system paths, with other BGP systems. This information determines autonomous-system connectivity, to prune routing loops, and to enforce autonomous-system-level policy decisions.

A router or switch running Cisco IOS does not select or use an IBGP route unless it has a route available to the next-hop router and it has received synchronization from an IGP (unless IGP synchronization is disabled). When multiple routes are available, BGP bases its path selection on attribute values. See the "Configuring BGP Decision Attributes" section for information about BGP attributes.

BGP Version 4 supports classless interdomain routing (CIDR), so you can reduce the size of your routing tables by creating aggregate routes, resulting in supernets. CIDR eliminates the network classes within BGP and supports the advertising of IP prefixes.

These sections contain this configuration information:

•Default BGP Configuration

•Enabling BGP Routing

•Managing Routing Policy Changes

•Configuring BGP Decision Attributes

•Configuring BGP Filtering with Route Maps

•Configuring BGP Filtering by Neighbor

•Configuring Prefix Lists for BGP Filtering

•Configuring BGP Community Filtering

•Configuring BGP Neighbors and Peer Groups

•Configuring Aggregate Addresses

•Configuring Routing Domain Confederations

•Configuring BGP Route Reflectors

•Configuring Route Dampening

•Monitoring and Maintaining BGP

For detailed descriptions of BGP configuration, see the "Configuring BGP" chapter in the "IP Routing Protocols" part of the Cisco IOS IP Configuration Guide, Release 12.2. For details about specific commands, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

For a list of BGP commands that are visible but not supported by the switch, see "Unsupported Commands in Cisco IOS Release 12.2(58)SE."

Default BGP Configuration

Table 39-9 shows the default BGP configuration. For the defaults for all characteristics, see the specific commands in the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

Nonstop Forwarding Awareness

The BGP NSF awareness feature is supported for IPv4 in the IP services feature set. To enable this feature with BGP routing, you need to enable graceful restart. When the neighboring router is NSF-capable, and this feature is enabled, the Layer 3 switch continues to forward packets from the neighboring router during the interval between the primary route processor in a router failure and the backup route processor take-over, or while you manually reload the primary route processor for a nondisruptive software upgrade.

For more information, see the "BGP Nonstop Forwarding (NSF) Awareness" section of the Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4.

Enabling BGP Routing

To enable BGP routing, you establish a BGP routing process and define the local network. Because BGP must fully recognize the relationships with its neighbors, you must also specify a BGP neighbor.

BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same autonomous system; external neighbors are in different autonomous systems. External neighbors are usually adjacent to each other and share a subnet, but internal neighbors can be anywhere in the same autonomous system.

The switch supports the use of private autonomous-system numbers, usually assigned by service providers and given to systems whose routes are not advertised to external neighbors. The private autonomous-system numbers are from 64512 to 65535. You can configure external neighbors to remove private autonomous-system numbers from the autonomous-system path by using the neighbor remove-private-as router configuration command. Then when an update is passed to an external neighbor, if the autonomous-system path includes any private autonomous-system numbers, these numbers are dropped.

If your autonomous system will be passing traffic through it from another autonomous system to a third autonomous system, it is important to be consistent about the routes it advertises. If BGP advertised a route before all routers in the network learned about the route through the IGP, the autonomous system might receive traffic that some routers could not yet route. Therefore, BGP must wait until the IGP has propagated information across the autonomous system so that BGP synchronizes with the IGP. Synchronization is enabled by default. If your autonomous system does not pass traffic from one autonomous system to another autonomous system, or if all routers in your autonomous systems are running BGP, you can disable synchronization, which allows your network to carry fewer routes in the IGP and allows BGP to converge more quickly.

Note To enable BGP, the switch or stack master must be running the IP services feature set.

Beginning in privileged EXEC mode, follow these steps to enable BGP routing, establish a BGP routing process, and specify a neighbor:

Graceful Restart Capability: advertised and received

Graceful Restart Capability: advertised

Use the no router bgp autonomous-system global configuration command to remove a BGP autonomous system. Use the no network network-number router configuration command to remove the network from the BGP table. Use the no neighbor {ip-address | peer-group-name} remote-as number router configuration command to remove a neighbor. Use the no neighbor {ip-address | peer-group-name} remove-private-as router configuration command to include private autonomous-system numbers in updates to a neighbor. Use the synchronization router configuration command to re-enable synchronization.

These examples show how to configure BGP on the routers in Figure 39-5.

Router A:

Switch(config)# router bgp 100

Switch(config-router)# neighbor 129.213.1.1 remote-as 200

Router B:

Switch(config)# router bgp 200

Switch(config-router)# neighbor 129.213.1.2 remote-as 100

Switch(config-router)# neighbor 175.220.1.2 remote-as 200

Router C:

Switch(config)# router bgp 200

Switch(config-router)# neighbor 175.220.212.1 remote-as 200

Switch(config-router)# neighbor 192.208.10.1 remote-as 300

Router D:

Switch(config)# router bgp 300

Switch(config-router)# neighbor 192.208.10.2 remote-as 200

To verify that BGP peers are running, use the show ip bgp neighbors privileged EXEC command. This is the output of this command on Router A:

Switch# show ip bgp neighbors

BGP neighbor is 129.213.1.1, remote AS 200, external link

 BGP version 4, remote router ID 175.220.212.1

 BGP state = established, table version = 3, up for 0:10:59

 Last read 0:00:29, hold time is 180, keepalive interval is 60 seconds

 Minimum time between advertisement runs is 30 seconds

 Received 2828 messages, 0 notifications, 0 in queue

 Sent 2826 messages, 0 notifications, 0 in queue

 Connections established 11; dropped 10

Anything other than BGP state = established means that the peers are not running. The remote router ID is the highest IP address on that router (or the highest loopback interface). Each time the table is updated with new information, the table version number increments. A table version number that continually increments means that a route is flapping, causing continual routing updates.

For exterior protocols, a reference to an IP network from the network router configuration command controls only which networks are advertised. This is in contrast to Interior Gateway Protocols (IGPs), such as EIGRP, which also use the network command to specify where to send updates.

For detailed descriptions of BGP configuration, see the "IP Routing Protocols" part of the Cisco IOS IP Configuration Guide, Release 12.2. For details about specific commands, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2. See "Unsupported Commands in Cisco IOS Release 12.2(58)SE," for a list of BGP commands that are visible but not supported by the switch.

Managing Routing Policy Changes

Routing policies for a peer include all the configurations that might affect inbound or outbound routing table updates. When you have defined two routers as BGP neighbors, they form a BGP connection and exchange routing information. If you later change a BGP filter, weight, distance, version, or timer, or make a similar configuration change, you must reset the BGP sessions so that the configuration changes take effect.

There are two types of reset, hard reset and soft reset. Cisco IOS Releases 12.1 and later support a soft reset without any prior configuration. To use a soft reset without preconfiguration, both BGP peers must support the soft-route refresh capability, which is advertised in the OPEN message sent when the peers establish a TCP session. A soft reset allows the dynamic exchange of route refresh requests and routing information between BGP routers and the subsequent re-advertisement of the respective outbound routing table.

•When soft reset generates inbound updates from a neighbor, it is called dynamic inbound soft reset.

•When soft reset sends a set of updates to a neighbor, it is called outbound soft reset.

A soft inbound reset causes the new inbound policy to take effect. A soft outbound reset causes the new local outbound policy to take effect without resetting the BGP session. As a new set of updates is sent during outbound policy reset, a new inbound policy can also take effect.

Table 39-10 lists the advantages and disadvantages hard reset and soft reset.

Beginning in privileged EXEC mode, follow these steps to learn if a BGP peer supports the route-refresh capability and to reset the BGP session:

Configuring BGP Decision Attributes

When a BGP speaker receives updates from multiple autonomous systems that describe different paths to the same destination, it must choose the single best path for reaching that destination. The selected path is then entered in the BGP routing table and propagated to its neighbors. The decision is based on the value of attributes that the update contains and other BGP-configurable factors.

When a BGP peer learns two EBGP paths for a prefix from a neighboring autonomous system, it chooses the best path and inserts that path in the IP routing table. If BGP multipath support is enabled and the EBGP paths are learned from the same neighboring autonomous systems, instead of a single best path, multiple paths are entered in the IP routing table. Then, during packet switching, per-packet or per-destination load-balancing is performed among the multiple paths. The maximum-paths router configuration command controls the number of paths allowed.

These factors summarize the order in which BGP evaluates the attributes for choosing the best path:

1. If the path specifies a next hop that is inaccessible, drop the update. The BGP next-hop attribute, automatically determined by the software, is the IP address of the next hop to be used to reach a destination. For EBGP, this is usually the IP address of the neighbor specified by the neighbor remote-as router configuration command. You can disable next-hop processing by using route maps or the neighbor next-hop-self router configuration command.

2. Prefer the path with the largest weight (a Cisco-proprietary parameter). The weight attribute is local to the router and not propagated in routing updates. By default, the weight attribute is 32768 for paths that the router originates and zero for other paths. Routes with the largest weight are preferred. You can use access lists, route maps, or the neighbor weight router configuration command to set weights.

3. Prefer the route with the highest local preference. Local preference is part of the routing update and is exchanged among routers in the same autonomous system. The default value of the local preference attribute is 100. You can set local preference by using the bgp default local-preference router configuration command or by using a route map.

4. Prefer the route that was originated by BGP running on the local router.

5. Prefer the route with the shortest autonomous-system path.

6. Prefer the route with the lowest origin type. An interior route or IGP is lower than a route learned by EGP, and an EGP-learned route is lower than one of unknown origin or learned in another way.

7. Prefer the route with the lowest multi-exit discriminator (MED) metric attribute if the neighboring autonomous system is the same for all routes considered. You can configure the MED by using route maps or by using the default-metric router configuration command. When an update is sent to an IBGP peer, the MED is included.

8. Prefer the external (EBGP) path over the internal (IBGP) path.

9. Prefer the route that can be reached through the closest IGP neighbor (the lowest IGP metric). This means that the router prefers the shortest internal path within the autonomous system to reach the destination (the shortest path to the BGP next hop).

10. If these conditions are all true, insert the route for this path into the IP routing table:

•Both the best route and this route are external.

•Both the best route and this route are from the same neighboring autonomous system.

•maximum-paths is enabled.

11. If multipath is not enabled, prefer the route with the lowest IP address value for the BGP router ID. The router ID is usually the highest IP address on the router or the loopback (virtual) address, but might be implementation-specific.

Beginning in privileged EXEC mode, follow these steps to configure some decision attributes:

Use the no form of each command to return to the default state.

Configuring BGP Filtering with Route Maps

Within BGP, route maps can control and modify routing information and t define the conditions by which routes are redistributed between routing domains. See the "Using Route Maps to Redistribute Routing Information" section for more information about route maps. Each route map has a name that identifies the it (map tag) and an optional sequence number.

Beginning in privileged EXEC mode, follow these steps to use a route map to disable next-hop processing:

Use the no route-map map-tag command to delete the route map. Use the no set ip next-hop ip-address command to re-enable next-hop processing.

Configuring BGP Filtering by Neighbor

You can filter BGP advertisements by using autonomous-system-path filters, such as the as-path access-list global configuration command and the neighbor filter-list router configuration command. You can also use access lists with the neighbor distribute-list router configuration command. Distribute-list filters are applied to network numbers. See the "Controlling Advertising and Processing in Routing Updates" section for information about the distribute-list command.

You can use route maps on a per-neighbor basis to filter updates and to modify various attributes. You can apply a route map to either inbound or outbound updates. Only the routes that pass the route map are sent or accepted in updates. On both inbound and outbound updates, matching is supported based on autonomous-system path, community, and network numbers. Autonomous-system-path matching requires the match as-path access-list route-map command, community-based matching requires the match community-list route-map command, and network-based matching requires the ip access-list global configuration command.

Beginning in privileged EXEC mode, follow these steps to apply a per-neighbor route map:

Use the no neighbor distribute-list command to remove the access list from the neighbor. Use the no neighbor route-map map-tag router configuration command to remove the route map from the neighbor.

Another filtering method is to specify an access list filter on both incoming and outbound updates, based on the BGP autonomous system paths. Each filter is an access list based on regular expressions. (See the "Regular Expressions" appendix in the Cisco IOS Dial Technologies Command Reference, Release 12.2 for more information on forming regular expressions.) To use this method, define an autonomous-system-path access list, and apply it to updates to and from particular neighbors.

Beginning in privileged EXEC mode, follow these steps to configure BGP autonomous-system-path filtering:

Configuring Prefix Lists for BGP Filtering

You can use prefix lists as an alternative to access lists in many BGP route filtering commands, including the neighbor distribute-list router configuration command. The advantages of using prefix lists include performance improvements in loading and lookup of large lists, incremental update support, easier CLI configuration, and greater flexibility.

Prefix-list filtering matches the prefixes of routes with those listed in the prefix list, as when matching access lists. When there is a match, the route is used. Whether a prefix is permitted or denied is based upon these rules:

•An empty prefix list permits all prefixes.

•An implicit deny is assumed if a prefix does not match any entries in a prefix list.

•When multiple entries of a prefix list match a prefix, the sequence number of a prefix-list entry identifies the entry with the lowest sequence number.

By default, sequence numbers are generated automatically and incremented in units of five. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry. You can specify sequence values in any increment. If you specify increments of one, you cannot insert additional entries into the list. If you choose very large increments, you might run out of values.

You do not need to specify a sequence number when removing a configuration entry. Show commands include the sequence numbers in their output.

Before using a prefix list in a command, you must set up the prefix list. Beginning in privileged EXEC mode, follow these steps to create a prefix list or to add an entry to a prefix list: