Cisco ME 3400E Ethernet Access Switch Software Configuration Guide, Release 12.2(44)EY

Configuring IP Unicast Routing

This chapter describes how to configure IP Version 4 (IPv4) unicast routing on the Cisco ME 3400EEthernet Access switch.

Note Routing is supported only on switches that are running the metro IP access image.

For more detailed IPv4 unicast configuration information, see the Cisco IOS IP Configuration Guide, Release 12.2 For complete syntax and usage information for the commands used in this chapter, see these 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 IPv4 Unicast Routing

•Configuring RIP

•Configuring OSPF

•Configuring EIGRP

•Configuring BGP

•Configuring 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 to allocate system resources to maximize the number of unicast routes allowed, you should use the sdm prefer default global configuration command to set the Switch Database Management (sdm) feature to balance resource. The Layer-2 template does not support routing and forces any routing to be done through software. This overloads the CPU and severely degrades routing performance. For more information on the SDM templates, see Chapter 7, "Configuring SDM Templates" or see the sdm prefer command in the command reference for this release.

Understanding IP Routing

In an IP network, each subnetwork is mapped to an individual VLAN. 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 36-1 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 36-1 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 checks 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.

Types of Routing

Routers and Layer 3 switches can route packets in three different ways:

•By using default routing—sending traffic with a destination unknown to the router to a default outlet or destination.

•By using preprogrammed static routes for the traffic

Static unicast routing forwards packets from predetermined ports through a single path into and out of a network. Static routing does not automatically respond to changes in the network and therefore, might result in unreachable destinations.

•By dynamically calculating routes by using a routing protocol

Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding traffic. Routing protocols supported by the switch are Routing Information Protocol (RIP), Border Gateway Protocol (BGP), Open Shortest Path First (OSPF) protocol, Enhanced IGRP (EIGRP), and System-to-Intermediate System (IS-IS).

Steps for Configuring Routing

By default, IPv4 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

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 on page 35-14.

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.

Note 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 support IPv4 routing, use the sdm prefer default global configuration command.

Configuring IPv4g routing consists of several main procedures:

•To support VLAN interfaces, create and configure VLANs on the switch, and assign VLAN membership to Layer 2 interfaces. For more information, see Chapter 12, "Configuring VLANs."

•Configure Layer 3 interfaces.

•Enable IPv4 routing on the switch.

•Assign IPv4 addresses to the Layer 3 interfaces.

•Enable selected routing protocols on the switch.

•Configure routing protocol parameters (optional).

Configuring IP Addressing

IP routing requires that Layer 3 network interfaces are assigned IP addresses to enable the interfaces and to allow communication with the hosts on 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

Assigning IP Addresses to Network Interfaces

An IP address identifies a location to which IP packets can be sent. 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

Subnetting with a subnet address of zero is strongly discouraged 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 it is discouraged, you can enable the use of 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 disable the use of subnet zero.

Classless Routing

By default, classless routing behavior is enabled on the switch when it 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 consists of contiguous blocks of Class C address spaces used to simulate a single, larger address space and is designed to relieve the pressure on the rapidly depleting Class B address space.

In Figure 36-2, 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 36-2 IP Classless Routing

In Figure 36-3, 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 36-3 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 and have the switch forward packets destined for a subnet of a network with no network default route to the best supernet route possible, 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 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. To communicate with a device on Ethernet, 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 can use these forms of address resolution:

•Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses. Taking 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.

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. Optionally, 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 methods 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, and the host that sent the request 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 sends an IP Control Message Protocol (ICMP) redirect message back, defining which local router the host should use. The switch caches the redirect messages and forwards each packet as efficiently as possible. A limitation of this method is that there is no means of detecting when the default router has gone down 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 using IRDP. IRDP allows hosts 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 listen to 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 further 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 optionally 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, so it is important to 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 two kinds of broadcasting:

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

•A flooded broadcast packet is 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 23, "Configuring Port-Based Traffic Control."

Routers provide some protection from broadcast storms by limiting their 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 modern IP implementations, you can set the address to be used as the broadcast address. The switch supports several addressing schemes for forwarding broadcast messages.

•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 not forwarded; they are dropped to make routers less susceptible to denial-of-service attacks. You can enable forwarding of IP-directed broadcasts on an interface where 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 list to control which broadcasts are forwarded. Only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts. For more information on access lists, see Chapter 32, "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 broadcast to physical broadcasts. Use the no ip forward-protocol global configuration command to remove a protocol or port.

Forwarding UDP Broadcast Packets and Protocols

User Datagram Protocol (UDP) is an IP host-to-host layer protocol that provides a low-overhead, connectionless session between two end systems and does not provide for acknowledgment of received datagrams. Network hosts occasionally 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 are normally not forwarded. You can configure an interface on a router to forward certain classes of broadcasts 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 (ND) protocol, which is used by older diskless Sun workstations and the network security protocol SDNS.

By default, both UDP and ND 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 UDP broadcast packets on an interface and 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 most popular 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, bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured on an interface, the interface can receive broadcasts but it never forwards the 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 used to consider packet forwarding 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, ND, 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 two.

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 so it might change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.

When a flooded UDP datagram is sent out an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, 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.

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.

Enabling IPv4 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 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 up parameters for the selected routing protocols as described in these sections:

•Configuring RIP

•Configuring OSPF

•Configuring EIGRP

•Configuring BGP

•Configuring ISO CLNS Routing

•Configuring Multi-VRF CE

•Configuring Protocol-Independent Features (optional)

Configuring RIP

The Routing Information Protocol (RIP) is an interior gateway protocol (IGP) used 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.

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 the non-updating 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. A directly connected network has a hop count of zero; a network with a hop count of 16 is unreachable. This small range (0 to 15) 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, but is treated by RIP as a network to implement default routing. 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 Split Horizon

Default RIP Configuration

Configuring Basic RIP Parameters

To configure RIP, you enable RIP routing for a network and optionally configure other parameters. On the Cisco ME 3400E switch, RIP configuration commands are ignored 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. Use the show ip rip database privileged EXEC command to display summary address entries in the RIP database.

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 two 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 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 when links are broken.

Note In general, Cisco does 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 Summary Addresses

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.

Note If split horizon is enabled, neither autosummary nor interface IP summary addresses are advertised.

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 10.2.0.0 overrides the autosummary address of 10.0.0.0 so that 10.2.0.0 is advertised out interface Gigabit Ethernet port 2, and 10.0.0.0 is not advertised. If the interface is in Layer 2 mode (the default), you must enter a 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 gi0/2

Switch(config-if)# no switchport

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 OSPF

Open Shortest Path First (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.

This section briefly describes how to configure O SPF. 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 multiaccess (NBMA), or point-to-point networks. Broadcast and nonbroadcast networks can also be configured as point-to-multipoint networks. The switch supports all these network types.

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, this means that 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

•Nonstop Forwarding Awareness

•Configuring OSPF Interfaces

•Configuring OSPF Network Types

•Configuring OSPF Area Parameters

•Configuring Other OSPF Parameters

•Changing LSA Group Pacing

•Configuring a Loopback Interface

•Monitoring OSPF

Default OSPF Configuration

Nonstop Forwarding Awareness

The OSPF NSF Awareness feature is supported for IPv4 in the metro IP access image. 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 (RP) in a router crashing and the backup RP taking over, or while the primary RP is manually reloaded for a non-disruptive software upgrade.

This feature cannot be disabled. For more information on this feature see the OSPF Nonstop Forwarding (NSF) Awareness Feature Guide at this URL:

http://cisco.com/en/US/products/sw/iosswrel/ps1839/products_white_paper09186a0080153edd.shtml

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 privileged EXEC mode, follow these steps to enable OSPF:

To terminate 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 number 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.

Note The ip ospf interface configuration commands are all optional.

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 return to the default value.

Configuring OSPF Network Types

OSPF classifies different media into the three types of networks by default:

•Broadcast networks (Ethernet, Token Ring, and FDDI)

•Nonbroadcast multiaccess (NBMA) networks (Switched Multimegabit Data Service [SMDS], Frame Relay, and X.25)

•Point-to-point networks (High-Level Data Link Control [HDLC], PPP)

You can also configure network interfaces as either a broadcast or an NBMA network and as point-to point or point-to-multipoint, regardless of the default media type.

Configuring OSPF for Nonbroadcast Networks

Because many routers might be attached to an OSPF network, a designated router is selected for the network. If broadcast capability is not configured in the network, the designated router selection requires special configuration parameters. You need to configure these parameters only for devices that are eligible to become the designated router or backup designated router (in other words, routers with a nonzero router priority value).

Beginning in privileged EXEC mode, follow these steps to configure routers that interconnect to nonbroadcast networks:

On point-to-multipoint, nonbroadcast networks, you then use the neighbor router configuration command to identify neighbors. Assigning a cost to a neighbor is optional.

Configuring Network Types for OSPF Interfaces

You can configure network interfaces as either broadcast or NBMA and as point-to point or point-to-multipoint, regardless of the default media type.

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface with one or more neighbors. On point-to-multipoint broadcast networks, specifying neighbors is optional. When you configure an interface as point-to-multipoint when the media does not support broadcast, you should use the neighbor command to identify neighbors.

Beginning in privileged EXEC mode, follow these steps to configure OSPF network type for an interface:

Use the no form of the ip ospf network command to return to the default network type for the media.

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 (AS). An NSSA does not flood all LSAs from the core into the area, but can import AS external routes within the area by redistribution.

Route summarization is the consolidation of 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 in 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 Area Border Routers 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 (interarea), 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 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 out 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. Because loopback interfaces never fail, this 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 36-6 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

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 and allows all devices involved in a topology change to 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 expanding the network 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.

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 neighbor is alive and functioning. When this status is determined, the neighboring routers 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 multiaccess network that has multicast capabilities, 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. To ensure low convergence time, the reliable transport sends multicast packets quickly when there are unacknowledged packets pending.

•The DUAL finite state machine handles the decision process for all route computations. It tracks all routes advertised by all neighbors and 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 must occur to determine a new successor. The amount of time it takes to recompute the route affects the convergence time. When a topology change occurs, DUAL tests for feasible successors 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. Routing decisions are stored in the IP routing table. EIGRP also redistributes 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

•Configuring EIGRP Stub Routing

•Monitoring and Maintaining EIGRP

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.

Nonstop Forwarding Awareness

The EIGRP NSF Awareness feature is supported for IPv4 in the metro IP access image. 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 (RP) in a router failing and the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software upgrade.

This feature cannot be disabled. For more information on this feature, see the EIGRP Nonstop Forwarding (NSF) Awareness Feature Guide at this URL: http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a0080160010.html

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 AS number identifies the routes to other EIGRP routers and is used to tag routing information.
Step 3	network network-number	Associate networks with an EIGRP routing process. EIGRP sends updates to the interfaces in the specified networks.
Step 4	eigrp log-neighbor-changes	(Optional) Enable logging of EIGRP neighbor changes to monitor routing system stability.
Step 5	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 is not recommended without guidance from an experienced network designer.
Step 6	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 7	no auto-summary	(Optional) Disable automatic summarization of subnet routes into network-level routes.
Step 8	ip summary-address eigrp autonomous-system-number address mask	(Optional) Configure a summary aggregate.
Step 9	end	Return to privileged EXEC mode.
Step 10	show ip protocols	Verify your entries. For NSF awareness, the output shows: *** IP Routing is NSF aware *** EIGRP NSF enabled
Step 11	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 return the setting to the default value.

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	no shutdown	Enable the interface if necessary. By default, UNIs and ENIs are disabled and NNIs are enabled.
Step 4	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 5	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 6	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 7	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 8	no ip split-horizon eigrp autonomous-system-number	(Optional) Disable split horizon to allow route information to be advertised by a router out any interface from which that information originated.
Step 9	end	Return to privileged EXEC mode.
Step 10	show ip eigrp interface	Display which interfaces EIGRP is active on and information about EIGRP relating to those interfaces.
Step 11	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 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.

Configuring EIGRP Stub Routing

The EIGRP stub routing feature reduces resource utilization by moving routed traffic closer to the end user. 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.

Note EIGRP stub routing 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. If you try to configure multi-VRF-CE and EIGRP stub routing at the same time, the configuration is not allowed.

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 36-4, 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 switch A and C. Switch B does not advertise any routes learned from switch A (and the reverse).

Figure 36-4 EIGRP Stub Router Configuration

For more information about EIGRP stub routing, see "Configuring EIGRP Stub Routing" part of the Cisco IOS IP Configuration Guide, Volume 2 of 3: Routing Protocols, Release 12.2 from the Cisco.com page under Documentation > Cisco IOS Software > 12.2 Mainline > Configuration Guides.

Beginning in privileged EXEC mode, follow these steps to configure a remote or spoke router for EIGRP stub routing:

Enter the show ip eigrp neighbor detail privileged EXEC command from the distribution router to verify the configuration.

Monitoring and Maintaining EIGRP

You can delete neighbors from the neighbor table. You can also display various EIGRP routing statistics. Table 36-8 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 used to set up an interdomain routing system for loop-free exchanges of routing information between autonomous systems. Autonomous systems are made up of routers that operate under the same administration and that run Interior Gateway Protocols (IGPs), such as RIP or OSPF, within their boundaries and that interconnect 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 IOS 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 Appendix C, "Unsupported Commands in Cisco IOS Release 12.2(44)EY."

Routers that belong to the same autonomous system (AS) and that exchange BGP updates run internal BGP (IBGP), and routers that belong to different autonomous systems and that exchange 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 AS (IBGP). Figure 36-5 shows a network that is running both EBGP and IBGP.

Figure 36-5 EBGP, IBGP, and Multiple Autonomous Systems

Before exchanging information with an external AS, BGP ensures that networks within the AS can be reached by defining internal BGP peering among routers within the AS and by redistributing BGP routing information to IGPs that run within the AS, 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 for exchanging routing information are known as peers or neighbors. In Figure 36-5, 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 AS 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 there is an IGP running that allows the two neighbors to reach one another, IBGP peers do not have to be directly connected.

•All BGP speakers within an AS must establish a peer relationship with each other. That is, the BGP speakers within an AS must be fully meshed logically. BGP4 provides two techniques that reduce the requirement for a logical full mesh: confederations and route reflectors.

•AS 200 is a transit AS for AS 100 and AS 300—that is, AS 200 is used to transfer packets between AS 100 and AS 300.

BGP peers initially 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 AS paths, with other BGP systems. This information can be used to determine AS connectivity, to prune routing loops, and to enforce AS-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 concept of network classes within BGP and supports the advertising of IP prefixes.

These sections contain 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 Appendix C, "Unsupported Commands in Cisco IOS Release 12.2(44)EY."

Default BGP Configuration

Nonstop Forwarding Awareness

The BGP NSF Awareness feature is supported for IPv4 in the metro IP access image. 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 (RP) in a router failing and the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software upgrade.

For more information, see the BGP Nonstop Forwarding (NSF) Awareness Feature Guide at this URL:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a008015fede.html

Enabling BGP Routing

To enable BGP routing, you establish a BGP routing process and define the local network. Because BGP must completely 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 AS; 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 AS.

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

If your AS must pass traffic through it from another AS to a third AS, it is important to be consistent about the routes it advertises. If BGP advertises a route before all routers in the network learn about the route through the IGP, the AS might receive traffic that some routers can not yet route. To prevent this from happening, BGP must wait until the IGP has propagated information across the AS so that BGP is synchronized with the IGP. Synchronization is enabled by default. If your AS does not pass traffic from one AS to another AS, 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.

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 AS. 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 AS 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 36-5.

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 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 Appendix C, "Unsupported Commands in Cisco IOS Release 12.2(44)EY," 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. The switch supports a soft reset without any prior configuration when both BGP peers 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.

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 decision is based on the value of attributes that the update contains and other BGP-configurable factors. The selected path is entered into the BGP routing table and propagated to its neighbors.

When a BGP peer learns two EBGP paths for a prefix from a neighboring AS, 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, multiple paths are installed 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 that is going 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. 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 exchanged among routers in the same AS. 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 AS 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 AS 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 will prefer the shortest internal path within the AS 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, you can use route maps to control and to modify routing information and to 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 route map (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 AS-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. A route map can be applied 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 AS 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 method of filtering 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 path filtering: