For BGP sessions between neighbors to be established, BGP must be assigned a router ID. The router ID is sent to BGP peers in the OPEN message when a BGP session is established.

BGP attempts to obtain a router ID in the following ways (in order of preference):

• By means of the address configured using the bgp router-id command in router configuration mode.

• By using the highest IPv4 address on a loopback interface in the system if the router is booted with saved loopback address configuration.

• By using the primary IPv4 address of the first loopback address that gets configured if there are not any in the saved configuration.

If none of these methods for obtaining a router ID succeeds, BGP does not have a router ID and cannot establish any peering sessions with BGP neighbors. In such an instance, an error message is entered in the system log, and the show bgp summary command displays a router ID of 0.0.0.0. After BGP has obtained a router ID, it continues to use it even if a better router ID becomes available. This usage avoids unnecessary flapping for all BGP sessions. However, if the router ID currently in use becomes invalid (because the interface goes down or its configuration is changed), BGP selects a new router ID (using the rules described) and all established peering sessions are reset.

Note	We strongly recommend that the bgp router-id command is configured to prevent unnecessary changes to the router ID (and consequent flapping of BGP sessions).

In network design solutions where customer equipment is dual-homed and Fast Reroute is required, such as in EVPN and BGP PIC Edge solutions, the Route Distinguisher (RD) associated with each VRF must be unique per Provider Edge (PE) router. In other design scenarios, while it isn’t mandatory for the RD to be unique per PE, it is highly recommended to make it unique. This practice facilitates easier transitions to dual-homed solutions in the future.

There are few available options to keep unique RD per device:

• Manual configuration: You must manually assign a unique value per device in the network. For example, in this scenario:

  • Leaf (ToR) = RD 1

  • Edge DCI Gateway = RD 2

  • Remote PE = RD 3

• Use rd auto command under VRF. To assign a unique route distinguisher for each router, you must ensure that each router has a unique BGP router-id. If so, the rd auto command assigns a Type 1 route distinguisher to the VRF using the following format: ip-address:number. The IP address is specified by the BGP router-id statement and the number (which is derived as an unused index in the 0 to 65535 range) is unique across the VRFs.

Note	In a DCI deployment, for route re-originate with stitching-rt for a particular VRF, using the same Route Distinguisher (RD) between edge DCI gateway and MPLS-VPN PE or same RD between edge DCI gateway and Leaf (ToR) is not supported.

This table summarizes the BGP attributes and operators per attach points.

BGP routers typically receive multiple paths to the same destination. The BGP best-path algorithm determines the best path to install in the IP routing table and to use for forwarding traffic. This section describes the Cisco IOS XR software implementation of BGP best-path algorithm, as specified in Section 9.1 of the Internet Engineering Task Force (IETF) Network Working Group draft-ietf-idr-bgp4-24.txt document.

The BGP best-path algorithm implementation is in three parts:

• Part 1—Compares two paths to determine which is better.

• Part 2—Iterates over all paths and determines which order to compare the paths to select the overall best path.

• Part 3—Determines whether the old and new best paths differ enough so that the new best path should be used.

Note

The order of comparison determined by Part 2 is important because the comparison operation is not transitive; that is, if three paths, A, B, and C exist, such that when A and B are compared, A is better, and when B and C are compared, B is better, it is not necessarily the case that when A and C are compared, A is better. This nontransitivity arises because the multi exit discriminator (MED) is compared only among paths from the same neighboring autonomous system (AS) and not among all paths.

The BGP Update Groups feature separates BGP update generation from neighbor configuration. The BGP Update Groups feature introduces an algorithm that dynamically calculates BGP update group membership based on outbound routing policies. This feature does not require any configuration by the network operator. Update group-based message generation occurs automatically and independently.

When a change to the configuration occurs, the router automatically recalculates update group memberships and applies the changes.

For the best optimization of BGP update group generation, we recommend that the network operator keeps outbound routing policy the same for neighbors that have similar outbound policies. This feature contains commands for monitoring BGP update groups.

The cost community attribute is applied to internal routes by configuring the set extcommunity cost command in a route policy. The cost community set clause is configured with a cost community ID number (0–255) and cost community number (0–4294967295). The cost community number determines the preference for the path. The path with the lowest cost community number is preferred. Paths that are not specifically configured with the cost community number are assigned a default cost community number of 2147483647 (the midpoint between 0 and 4294967295) and evaluated by the best-path selection process accordingly. When two paths have been configured with the same cost community number, the path selection process prefers the path with the lowest cost community ID. The cost-extended community attribute is propagated to iBGP peers when extended community exchange is enabled.

The following commands include the route-policy keyword, which you can use to apply a route policy that is configured with the cost community set clause:

• aggregate-address

• redistribute

• network

Event notifications from the RIB are classified as critical and noncritical. Notifications for critical and noncritical events are sent in separate batches. BGP is notified when any of the following events occurs:

• Next hop becomes unreachable

• Next hop becomes reachable

• Fully recursed IGP metric to the next hop changes

• First hop IP address or first hop interface change

• Next hop becomes connected

• Next hop becomes unconnected

• Next hop becomes a local address

• Next hop becomes a nonlocal address

Note	Reachability and recursed metric events trigger a best-path recalculation.

However, a noncritical event is sent along with the critical events if the noncritical event is pending and there is a request to read the critical events.

• Critical events are related to the reachability (reachable and unreachable), connectivity (connected and unconnected), and locality (local and nonlocal) of the next hops. Notifications for these events are not delayed.

• Noncritical events include only the IGP metric changes. These events are sent at an interval of 3 seconds. A metric change event is batched and sent 3 seconds after the last one was sent.

BGP is notified when any of the following events occurs:

• Next hop becomes unreachable

• Next hop becomes reachable

• Fully recursed IGP metric to the next hop changes

• First hop IP address or first hop interface change

• Next hop becomes connected

• Next hop becomes unconnected

• Next hop becomes a local address

• Next hop becomes a nonlocal address

Note	Reachability and recursed metric events trigger a best-path recalculation.

The next-hop trigger delay for critical and noncritical events can be configured to specify a minimum batching interval for critical and noncritical events using the nexthop trigger-delay command. The trigger delay is address family dependent.

The BGP next-hop tracking feature allows you to specify that BGP routes are resolved using only next hops whose routes have the following characteristics:

• To avoid the aggregate routes, the prefix length must be greater than a specified value.

• The source protocol must be from a selected list, ensuring that BGP routes are not used to resolve next hops that could lead to oscillation.

This route policy filtering is possible because RIB identifies the source protocol of route that resolved a next hop as well as the mask length associated with the route. The nexthop route-policy command is used to specify the route-policy.

Next Hop as the IPv6 Address of Peering Interface

BGP can carry IPv6 prefixes over an IPv4 session. The next hop for the IPv6 prefixes can be set through a nexthop policy. In the event that the policy is not configured, the nexthops are set as the IPv6 address of the peering interface (IPv6 neighbor interface or IPv6 update source interface, if any one of the interfaces is configured).

If the nexthop policy is not configured and neither the IPv6 neighbor interface nor the IPv6 update source interface is configured, the next hop is the IPv4 mapped IPv6 address.

Scoped IPv4/VPNv4 Table Walk

To determine which address family to process, a next-hop notification is received by first de-referencing the gateway context associated with the next hop, then looking into the gateway context to determine which address families are using the gateway context. The IPv4 unicast and VPNv4 unicast address families share the same gateway context, because they are registered with the IPv4 unicast table in the RIB. As a result, both the global IPv4 unicast table and the VPNv4 table are is processed when an IPv4 unicast next-hop notification is received from the RIB. A mask is maintained in the next hop, indicating if whether the next hop belongs to IPv4 unicast or VPNv4 unicast, or both. This scoped table walk localizes the processing in the appropriate address family table.

Reordered Address Family Processing

The software walks address family tables based on the numeric value of the address family. When a next-hop notification batch is received, the order of address family processing is reordered to the following order:

• IPv4 tunnel

• VPNv4 unicast

• VPNv6 unicast

• IPv4 labeled unicast

• IPv4 unicast

• IPv4 MDT

• IPv6 unicast

• IPv6 labeled unicast

• IPv4 tunnel

• VPNv4 unicast

• IPv4 unicast

• IPv6 unicast

New Thread for Next-Hop Processing

The critical-event thread in the spkr process handles only next-hop, Bidirectional Forwarding Detection (BFD), and fast-external-failover (FEF) notifications. This critical-event thread ensures that BGP convergence is not adversely impacted by other events that may take a significant amount of time.

show, clear, and debug Commands

The show bgp nexthops command provides statistical information about next-hop notifications, the amount of time spent in processing those notifications, and details about each next hop registered with the RIB. The clear bgp nexthop performance-statistics command ensures that the cumulative statistics associated with the processing part of the next-hop show command can be cleared to help in monitoring. The clear bgp nexthop registration command performs an asynchronous registration of the next hop with the RIB.

The debug bgp nexthop command displays information on next-hop processing. The out keyword provides debug information only about BGP registration of next hops with RIB. The in keyword displays debug information about next-hop notifications received from RIB. The out keyword displays debug information about next-hop notifications sent to the RIB.

BGP NSR provides nonstop routing during the following events:

• Route processor switchover

• Process crash or process failure of BGP or TCP

  Note

  BGP NSR is enabled by default. Use the nsr disable command to turn off BGP NSR. The no nsr disable command can also be used to turn BGP NSR back on if it has been disabled. In case of process crash or process failure, NSR will be maintained only if nsr process-failures switchover command is configured. In the event of process failures of active instances, the nsr process-failures switchover configures failover as a recovery action and switches over to a standby route processor (RP) or a standby distributed route processor (DRP) thereby maintaining NSR. An example of the configuration command is RP/0/RSP0/CPU0:router(config) # nsr process-failures switchover The nsr process-failures switchover command maintains both the NSR and BGP sessions in the event of a BGP or TCP process crash. Without this configuration, BGP neighbor sessions flap in case of a BGP or TCP process crash. This configuration does not help if the BGP or TCP process is restarted in which case the BGP neighbors are expected to flap. When the l2vpn_mgr process is restarted, the NSR client (te-control) flaps between the Ready and Not Ready state. This is the expected behavior and there is no traffic loss.

During route processor switchover and In-Service System Upgrade (ISSU), NSR is achieved by stateful switchover (SSO) of both TCP and BGP.

NSR does not force any software upgrades on other routers in the network, and peer routers are not required to support NSR.

When a route processor switchover occurs due to a fault, the TCP connections and the BGP sessions are migrated transparently to the standby route processor, and the standby route processor becomes active. The existing protocol state is maintained on the standby route processor when it becomes active, and the protocol state does not need to be refreshed by peers.

Events such as soft reconfiguration and policy modifications can trigger the BGP internal state to change. To ensure state consistency between active and standby BGP processes during such events, the concept of post-it is introduced that act as synchronization points.

BGP NSR provides the following features:

• NSR-related alarms and notifications

• Configured and operational NSR states are tracked separately

• NSR statistics collection

• NSR statistics display using show commands

• XML schema support

• Auditing mechanisms to verify state synchronization between active and standby instances

• CLI commands to enable and disable NSR

#concept_305B2C3244D5404F9E6207098D05CA9E__ illustrates a simple iBGP configuration with three iBGP speakers (routers A, B, and C). Without route reflectors, when Router A receives a route from an external neighbor, it must advertise it to both routers B and C. Routers B and C do not readvertise the iBGP learned route to other iBGP speakers because the routers do not pass on routes learned from internal neighbors to other internal neighbors, thus preventing a routing information loop.

With route reflectors, all iBGP speakers need not be fully meshed because there is a method to pass learned routes to neighbors. In this model, an iBGP peer is configured to be a route reflector responsible for passing iBGP learned routes to a set of iBGP neighbors. In #concept_305B2C3244D5404F9E6207098D05CA9E__ , Router B is configured as a route reflector. When the route reflector receives routes advertised from Router A, it advertises them to Router C, and vice versa. This scheme eliminates the need for the iBGP session between routers A and C.

The internal peers of the route reflector are divided into two groups: client peers and all other routers in the autonomous system (nonclient peers). A route reflector reflects routes between these two groups. The route reflector and its client peers form a cluster. The nonclient peers must be fully meshed with each other, but the client peers need not be fully meshed. The clients in the cluster do not communicate with iBGP speakers outside their cluster.

#concept_305B2C3244D5404F9E6207098D05CA9E__ illustrates a more complex route reflector scheme. Router A is the route reflector in a cluster with routers B, C, and D. Routers E, F, and G are fully meshed, nonclient routers.

When the route reflector receives an advertised route, depending on the neighbor, it takes the following actions:

• A route from an external BGP speaker is advertised to all clients and nonclient peers.

• A route from a nonclient peer is advertised to all clients.

• A route from a client is advertised to all clients and nonclient peers. Hence, the clients need not be fully meshed.

Along with route reflector-aware BGP speakers, it is possible to have BGP speakers that do not understand the concept of route reflectors. They can be members of either client or nonclient groups, allowing an easy and gradual migration from the old BGP model to the route reflector model. Initially, you could create a single cluster with a route reflector and a few clients. All other iBGP speakers could be nonclient peers to the route reflector and then more clusters could be created gradually.

An autonomous system can have multiple route reflectors. A route reflector treats other route reflectors just like other iBGP speakers. A route reflector can be configured to have other route reflectors in a client group or nonclient group. In a simple configuration, the backbone could be divided into many clusters. Each route reflector would be configured with other route reflectors as nonclient peers (thus, all route reflectors are fully meshed). The clients are configured to maintain iBGP sessions with only the route reflector in their cluster.

Usually, a cluster of clients has a single route reflector. In that case, the cluster is identified by the router ID of the route reflector. To increase redundancy and avoid a single point of failure, a cluster might have more than one route reflector. In this case, all route reflectors in the cluster must be configured with the cluster ID so that a route reflector can recognize updates from route reflectors in the same cluster. All route reflectors serving a cluster should be fully meshed and all of them should have identical sets of client and nonclient peers.

By default, the clients of a route reflector are not required to be fully meshed and the routes from a client are reflected to other clients. However, if the clients are fully meshed, the route reflector need not reflect routes to clients.

As the iBGP learned routes are reflected, routing information may loop. The route reflector model has the following mechanisms to avoid routing loops:

• Originator ID is an optional, nontransitive BGP attribute. It is a 4-byte attributed created by a route reflector. The attribute carries the router ID of the originator of the route in the local autonomous system. Therefore, if a misconfiguration causes routing information to come back to the originator, the information is ignored.

• Cluster-list is an optional, nontransitive BGP attribute. It is a sequence of cluster IDs that the route has passed. When a route reflector reflects a route from its clients to nonclient peers, and vice versa, it appends the local cluster ID to the cluster-list. If the cluster-list is empty, a new cluster-list is created. Using this attribute, a route reflector can identify if routing information is looped back to the same cluster due to misconfiguration. If the local cluster ID is found in the cluster-list, the advertisement is ignored.

When there are multiple border BGP routers having reachability information heard over eBGP, if no local policy is applied, the border routers will choose their eBGP paths as best. They advertise that bestpath inside the ISP network. For a core router, there can be multiple paths to the same destination, but it will select only one path as best and use that path for forwarding. iBGP multipath load sharing adds the ability to enable load sharing among multiple equi-distant paths. Configuring multiple iBGP best paths enables a router to evenly share the traffic destined for a particular site. The iBGP Multipath Load Sharing feature functions similarly in a Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) with a service provider backbone.

For multiple paths to the same destination to be considered as multipaths, the following criteria must be met:

• All attributes must be the same. The attributes include weight, local preference, autonomous system path (entire attribute and not just length), origin code, Multi Exit Discriminator (MED), and Interior Gateway Protocol (iGP) distance.

• The next hop router for each multipath must be different.

Even if the criteria are met and multiple paths are considered multipaths, the BGP speaking router designates one of the multipaths as the best path and advertises this best path to its neighbors.

Note

Overwriting of next-hop calculation for multipath prefixes is not allowed. The next-hop-unchanged multipath command disables overwriting of next-hop calculation for multipath prefixes. The ability to ignore as-path onwards while computing multipath is added. The bgp multipath as-path ignore onwards command ignores as-path onwards while computing multipath.

When BGP is used as the provider edge (PE) or the customer edge (CE) routing protocol, the peering sessions are configured as external peering between the VPN provider autonomous system (AS) and the customer network autonomous system. The L3VPN iBGP PE-CE feature enables the PE and CE devices to exchange Border Gateway Protocol (BGP) routing information by peering as internal Border Gateway Protocol (iBGP) instead of the widely-used external BGP peering between the PE and the CE. This mechanism applies at each PE device where a VRF-based CE is configured as iBGP. This eliminates the need for service providers (SPs) to configure autonomous system override for the CE. With this feature enabled, there is no need to configure the virtual private network (VPN) sites using different autonomous systems.

The neighbor internal-vpn-client command enables PE devices to make an entire VPN cloud act as an internal VPN client to the CE devices. These CE devices are connected internally to the VPN cloud through the iBGP PE-CE connection inside the VRF. After this connection is established, the PE device encapsulates the CE-learned path into an attribute called ATTR_SET and carries it in the iBGP-sourced path throughout the VPN core to the remote PE device. At the remote PE device, this attribute is assigned with individual attributes and the source CE path is extracted and sent to the remote CE devices.

										+------------------------------+
          | Attr Flags (O|T) Code = 128  |
          +------------------------------+
          | Attr. Length (1 or 2 octets) |
          +------------------------------+
          | Origin AS (4 octets)         |
          +------------------------------+
          | Path attributes (variable)   |
          +------------------------------+

Origin AS is the AS of the VPN customer for which the ATTR_SET is generated. The minimum length of ATTR_SET is four bytes and the maximum is the maximum supported for a path attribute after taking into consideration the mandatory fields and attributes in the BGP update message. It is recommended that the maximum length is limited to 3500 bytes. ATTR_SET must not contain the following attributes: MP_REACH, MP_UNREACH, NEW_AS_PATH, NEW_AGGR, NEXT_HOP and ATTR_SET itself (ATTR_SET inside ATTR_SET). If these attributes are found inside the ATTR_SET, the ATTR_SET is considered invalid and the corresponding error handling mechanism is invoked.

Carrier supporting carrier (CSC) is a term used to describe a situation in which one service provider allows another service provider to use a segment of its backbone network. The service provider that provides the segment of the backbone network to the other provider is called the backbone carrier. The service provider that uses the segment of the backbone network is called the customer carrier.

A backbone carrier offers Border Gateway Protocol and Multiprotocol Label Switching (BGP/MPLS) VPN services. The customer carrier can be either:

• An Internet service provider (ISP) (By definition, an ISP does not provide VPN service.)

• A BGP/MPLS VPN service provider

You can configure a CSC network to enable BGP to transport routes and MPLS labels between the backbone carrier provider edge (PE) routers and the customer carrier customer edge (CE) routers using multiple paths. The benefits of using BGP to distribute IPv4 routes and MPLS label routes are:

• BGP takes the place of an Interior Gateway Protocol (IGP) and Label Distribution Protocol (LDP) in a VPN routing and forwarding (VRF) table. You can use BGP to distribute routes and MPLS labels. Using a single protocol instead of two simplifies the configuration and troubleshooting.

• BGP is the preferred routing protocol for connecting two ISPs, mainly because of its routing policies and ability to scale. ISPs commonly use BGP between two providers. This feature enables those ISPs to use BGP.

For detailed information on configuring MPLS VPN CSC with BGP, see the Implementing MPLS Layer 3 VPNs on module of the MPLS Configuration Guide.

The per VRF and per CE label for IPv6 feature makes it possible to save label space by allocating labels per default VRF or per CE nexthop.

All IPv6 Provider Edge (6PE) labels are allocated per prefix by default. Each prefix that belongs to a VRF instance is advertised with a single label, causing an additional lookup to be performed in the VRF forwarding table to determine the customer edge (CE) next hop for the packet.

However, use the label-allocation-mode command with the per-ce keyword or the per-vrf keyword to avoid the additional lookup on the PE router and conserve label space.

Use per-ce keyword to specify that the same label be used for all the routes advertised from a unique customer edge (CE) peer router. Use the per-vrf keyword to specify that the same label be used for all the routes advertised from a unique VRF.

Note	The label-allocation-mode command is deprecated from 7.4.1 release. The function of this command can be carried out using label mode command under configured address-family.

Cisco provides complete Internet Protocol Version 6 (IPv6) unicast capability.

An IPv6 unicast address is an identifier for a single interface, on a single node. A packet that is sent to a unicast address is delivered to the interface identified by that address. Cisco IOS XR software supports the following IPv6 unicast address types:

• Global aggregatable address

• Site-local address

• Link-local address

• IPv4-compatible IPv6 address

For more information on IPv6 unicast addressing, refer the IP Addresses and Services Configuration Guide.

Private autonomous system numbers (ASNs) are used by Internet Service Providers (ISPs) and customer networks to conserve globally unique AS numbers. Private AS numbers cannot be used to access the global Internet because they are not unique. AS numbers appear in eBGP AS paths in routing updates. Removing private ASNs from the AS path is necessary if you have been using private ASNs and you want to access the global Internet.

Public AS numbers are assigned by InterNIC and are globally unique. They range from 1 to 64511. Private AS numbers are used to conserve globally unique AS numbers, and they range from 64512 to 65535. Private AS numbers cannot be leaked to a global BGP routing table because they are not unique, and BGP best path calculations require unique AS numbers. Therefore, it might be necessary to remove private AS numbers from an AS path before the routes are propagated to a BGP peer.

External BGP (eBGP) requires that globally unique AS numbers be used when routing to the global Internet. Using private AS numbers (which are not unique) would prevent access to the global Internet. The remove and replace private AS Numbers from AS Path in BGP feature allows routers that belong to a private AS to access the global Internet. A network administrator configures the routers to remove private AS numbers from the AS path contained in outgoing update messages and optionally, to replace those numbers with the ASN of the local router, so that the AS Path length remains unchanged.

The ability to remove and replace private AS numbers from the AS Path is implemented in the following ways:

• The remove-private-as command removes private AS numbers from the AS path even if the path contains both public and private ASNs.

• The remove-private-as command removes private AS numbers even if the AS path contains only private AS numbers. There is no likelihood of a 0-length AS path because this command can be applied to eBGP peers only, in which case the AS number of the local router is appended to the AS path.

• The remove-private-as command removes private AS numbers even if the private ASNs appear before the confederation segments in the AS path.

• The replace-as command replaces the private AS numbers being removed from the path with the local AS number, thereby retaining the same AS path length.

The feature can be applied to neighbors per address family (address family configuration mode). Therefore, you can apply the feature for a neighbor in one address family and not on another, affecting update messages on the outbound side for only the address family for which the feature is configured.

Use show bgp neighbors and show bgp update-group commands to verify that the that private AS numbers were removed or replaced.

BGP routers typically receive multiple paths to the same destination. The BGP best-path algorithm determines the best path to install in the IP routing table and to use for forwarding traffic. The overall best path is selected based on various attributes. .

AS path is one of the attributes used for best path selection. By default, BGP always prefers the route with shortest AS path as the best path. The best path selected by BGP might have traffic engineering issues, like heavy traffic that leads to congestion. In such cases, you can alter the best path by replacing the AS path with custom values.

The following are the custom values you can use to replace the AS path:

• None: Use this option to modify an AS path as the shortest path in the network. When you choose this option, the AS path is replaced with a null or empty value. Use the replace as-path all none command to replace with none.

• Auto: Use this option to advertise the local AS number or the neigbor's AS number as the AS path. When you choose this option, AS path is replaced based on the route policy:

  • For inbound route policy, AS path is replaced with AS path of BGP neighbor from where the prefix is received.

  • For outbound route policy, AS path is replaced with the local AS number.

  Use the replace as-path all auto command to replace with auto.

• 'x': Use this option to replace AS path with any specified value. Use the replace as-path all 'x' command to replace with this option, where 'x' can be a single AS number or a sequence of AS numbers separated by space.

• Optionally, you can repeat replacing the AS path for a specified number of times. This option is supported only for the auto and 'x' parameters. Use the replace as-path all {auto | 'x'} [n] command to enable the repeat option.

• Optionally, you can use a parameter name along with the repeat option. The parameter name must be preceded with a “$.” You can attach the route policy with the parameter to a neighbor and specify the number of times the AS path replacement should be repeated. This opton allows you to apply the same route policy to different neighbors with different AS path values.

  Use the replace as-path all {auto | 'x'} [n] [parameter] command to enable the parameter along with repeat option.

You can replace the AS path for inbound eBGP, outbound eBGP, and outbound iBGP paths.

Note	For outbound eBGP paths, the AS number of the local router is always prepended to the replaced AS path.

Deployment Scenario

Consider a BGP network configured with AS paths. By default, BGP selects the route with shortest AS path to reach the destination. You can alter the default route by using the replace BGP AS path feature.

In the following figure, the network consists of BGP routers configured with AS Path values. To reach Server B, Server A typically selects Path B (via S1_1, S0), as the AS path value of S0 is shorter.

You may want to use Path A to reach the destination (via S1_1, S2_1, S1_2, S0), for traffic engineering purpose. For example, Path A may be less congested and is better than Path B. To use Path A, you can replace the AS path values with one of the following options:

• Replace AS path of Router S2_1 with a shorter value.

• Replace AS path of Router S0 with a longer value.

/*Configure route policy to replace AS path with none*/
Router(config)#hw-module profile stats ?
Router(config)# route-policy aspath-none
Router(config-rpl)# replace as-path all none
Router(config-rpl)# end-policy

/* Apply route policy to BGP neighbor */
Router(config)# router bgp 65530
Router(config-bgp)# neighbor 111.0.0.1
Router(config-bgp-nbr)# address-family ipv4 unicast
Router(config-bgp-nbr-af)# route-policy aspath-none in

/*Configure route policy to replace AS path with auto*/
Router(config)#route-policy aspath-auto
Router(config-rpl)# replace as-path all auto
Router(config-rpl)# end-policy


/* Apply route policy to BGP neighbor */
Router(config)# router bgp 65530
Router(config-bgp)# neighbor 111.0.0.1
Router(config-bgp-nbr)# address-family ipv4 unicast
Router(config-bgp-nbr-af)# route-policy aspath-auto out

/*Configure route policy to replace AS path with 'x'*/
Router(config)# route-policy aspath-str
Router(config-rpl)# replace as-path all '10 100 200 300'
Router(config-rpl)# end-policy


/* Apply route policy to BGP neighbor */
Router(config)# router bgp 1
Router(config-bgp)# neighbor 111.0.0.1
Router(config-bgp-nbr)# address-family ipv4 unicast
Router(config-bgp-nbr-af)# route-policy aspath-str in

/*Configure route policy to replace AS path with parameter ($n)*/
Router(config)# route-policy aspath-par($n)
Router(config-rpl)# replace as-path all '45 55' $n
Router(config-rpl)# end-policy


/* Apply route policy to BGP neighbor */
Router(config)# router bgp 1
Router(config-bgp)# neighbor 111.0.0.1
Router(config-bgp-nbr)# address-family ipv4 unicast
Router(config-bgp-nbr-af)# route-policy aspath-par(6) in

Router# show bgp
Network               Next Hop            Metric LocPrf Weight Path
*> 192.168.3.0/24     192.168.3.1                     0      0    i

Router# show bgp
Network            Next Hop            Metric LocPrf Weight Path
*> 111.0.0.2/32    200.0.0.5                       0         40 i    

Router# show bgp
Network            Next Hop            Metric LocPrf Weight Path
*>111.0.0.2/32      200.0.0.5                      0 10 100 200 300 i

Router# show bgp
Network            Next Hop            Metric LocPrf Weight Path
*>111.0.0.8/32      200.0.0.5                      0        45 55 45 55 45 55 45 55 45 55 45 55 i

The BGP UPDATE message error handling changes BGP behavior in handling error UPDATE messages to avoid session reset. Based on the approach described in IETF IDR I-D:draft-ietf-idr-error-handling, the Cisco IOS XR BGP UPDATE Message Error handling implementation classifies BGP update errors into various categories based on factors such as, severity, likelihood of occurrence of UPDATE errors, or type of attributes. Errors encountered in each category are handled according to the draft. Session reset will be avoided as much as possible during the error handling process. Error handling for some of the categories are controlled by configuration commands to enable or disable the default behavior.

According to the base BGP specification, a BGP speaker that receives an UPDATE message containing a malformed attribute is required to reset the session over which the offending attribute was received. This behavior is undesirable as a session reset would impact not only routes with the offending attribute, but also other valid routes exchanged over the session.

When a router receives a malformed update packet, an ios_msg of type ROUTING-BGP-3-MALFORM_UPDATE is printed on the console. This is rate limited to 1 message per minute across all neighbors. For malformed packets that result in actions "Discard Attribute" (A5) or "Local Repair" (A6), the ios_msg is printed only once per neighbor per action. This is irrespective of the number of malformed updates received since the neighbor last reached an "Established" state.

%ROUTING-BGP-3-MALFORM_UPDATE : Malformed UPDATE message received from neighbor 13.0.3.50 - message length 90 bytes,
 error flags 0x00000840, action taken "TreatAsWithdraw". 
Error details: "Error 0x00000800, Field "Attr-missing", Attribute 1 (Flags 0x00, Length 0), Data []"

[4843.46]RP/0/0/CPU0:Aug 21 17:06:17.919 : bgp[1037]: %ROUTING-BGP-5-UPDATE_FILTERED : 
One or more attributes were filtered from UPDATE message received from neighbor 40.0.101.1 - message length 173 bytes,
 action taken "DiscardAttr".
 Filtering details: "Attribute 16 (Flags 0xc0): Action "DiscardAttr"". NLRIs: [IPv4 Unicast] 88.2.0.0/17

[391.01]RP/0/0/CPU0:Aug 20 19:41:29.243 : bgp[1037]: %ROUTING-BGP-5-UPDATE_FILTERED :
 One or more attributes were filtered from UPDATE message received from neighbor 40.0.101.1 - message length 166 bytes,
 action taken "TreatAsWdr".
 Filtering details: "Attribute 4 (Flags 0xc0): Action "TreatAsWdr"". NLRIs: [IPv4 Unicast] 88.2.0.0/17

The Border Gateway Protocol-Routing Information Base (BGP-RIB) feedback mechanism for update generation feature avoids premature route advertisements and subsequent packet loss in a network. This mechanism ensures that routes are installed locally, before they are advertised to a neighbor.

BGP waits for feedback from RIB indicating that the routes that BGP installed in RIB are installed in forwarding information base (FIB) before BGP sends out updates to the neighbors. RIB uses the the BCDL feedback mechanism to determine which version of the routes have been consumed by FIB, and updates the BGP with that version. BGP will send out updates of only those routes that have versions up to the version that FIB has installed. This selective update ensures that BGP does not send out premature updates resulting in attracting traffic even before the data plane is programmed after router reload, LC OIR, or flap of a link where an alternate path is made available.

To configure BGP to wait for feedback from RIB indicating that the routes that BGP installed in RIB are installed in FIB, before BGP sends out updates to neighbors, use the update wait-install command in router address-family IPv4 or router address-family VPNv4 configuration mode. The show bgp , show bgp neighbors , and show bgp process performance-statistics commands display the information from update wait-install configuration.

When BGP forwards traffic, it waits for feedback from the RIB until the RIB is ready to forward traffic. Once the RIB is ready, BGP sends the route updates to the BGP neighbors and peer-groups. Advertising routes before the RIB is synchronized in the FIB results in traffic loss. To avoid this problem, the router must delay the BGP start-up process to delay the BGP update generation so that no traffic loss happens.

To accomplish this, you must configure the update wait-install delay startup command to delay the generation of BGP updates. The show bgp process command displays the delay of the BGP process update since the last router reload.

This feature allows you to configure the minimum and maximum delay periods. The range of the delay is from 1 second to 600 seconds. As a result, network traffic loss is avoided.

Restrictions

This feature is applicable for the following Address Family Indicators (AFIs):

• IPv4 unicast

• IPv6 unicast

• VPNv4 unicast

• VPNv6 unicast

The solution allows disabling the Martian check for these IP address prefixes:

• IPv4 address prefixes

  • 0.0.0.0/8

  • 127.0.0.0/8

  • 224.0.0.0/4

• IPv6 address prefixes

  • ::

  • ::0002 - ::ffff

  • ::ffff:a.b.c.d

  • fe80:xxxx

  • ffxx:xxxx

BGP supports aggregating dmz-link bandwidth values of external BGP (eBGP) multipaths when advertising the route to interior BGP (iBGP) peer.

There is no explicit command to aggregate bandwidth. The bandwidth is aggregated if following conditions are met:

• The network has multipaths and all the multipaths have link-bandwidth values.

• The next-hop attribute set to next-hop-self. The next-hop attribute for all routes advertised to the specified neighbor to the address of the local router.

• There is no out-bound policy configured that might change the dmz-link bandwidth value.

• If the dmz-link bandwidth value is not known for any one of the multipaths (eBGP or iBGP), the dmz-link value for all multipaths including the best path is not downloaded to routing information base (RIB).

• The dmz-link bandwidth value of iBGP multipath is not considered during aggregation.

• The route that is advertised with aggregate value can be best path or add-path.

• Add-path does not qualify for DMZ link bandwidth aggregation as next hop is preserved. Configuring next-hop-self for add-path is not supported.

• For VPNv4 and VPNv6 afi, if dmz link-bandwidth value is configured using outbound route-policy, specify the route table or use the additive keyword. Else, this will lead to routes not imported on the receiving end of the peer.

extcommunity-set bandwidth dmz_ext
   1:8000
 end-set
 !
 route-policy dmz_rp_vpn
   set extcommunity bandwidth dmz_ext additive     <<< 'additive' keyword.
   pass
 end-policy

/* Delete all the extended communities. */
Router(config)# route-policy dmz_del_all 
Router(config-rpl)# delete extcommunity bandwidth all
Router(config-rpl)# pass
Router(config-rpl)# end-policy

/* Delete only the extended communities that match an extended community mentioned in the list. */ 
Router(config)# route-policy dmz_CE1_del_non_match
Router(config-rpl)# if destination in (10.9.9.9/32) then 
Router(config-rpl-if)# delete extcommunity bandwidth in (10:7000)
Router(config-rpl-if)# endif
Router(config-rpl)# pass
Router(config-rpl)# end-policy

/* Delete all the extended communities. */
Router(config)# route-policy dmz_del_param2($a,$b)
Router(config-rpl)# if destination in (10.9.9.9/32) then 
Router(config-rpl-if)# delete extcommunity bandwidth in ($a:$b)
Router(config-rpl-if)# endif
Router(config-rpl)# pass
Router(config-rpl)# end-policy

Router# show bgp 10.9.9.9/32
Fri Aug 27 13:15:05.833 EDT
BGP routing table entry for 10.9.9.9/32
Versions:
Process bRIB/RIB SendTblVer
Speaker 15 15
Last Modified: Aug 27 13:06:45.000 for 00:08:21
Paths: (3 available, best #1)
Advertised IPv4 Unicast paths to peers (in unique update groups):
13.13.13.5
Path #1: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
13.13.13.5
10
10.10.10.1 from 10.10.10.1 (192.168.0.1)
Origin incomplete, metric 0, localpref 100, valid, external, best, group-best, multipath
Received Path ID 0, Local Path ID 1, version 15
Extended community: LB:10:48
Origin-AS validity: (disabled)
Path #2: Received by speaker 0
Not advertised to any peer
10
11.11.11.3 from 11.11.11.3 (192.168.0.3)
Origin incomplete, metric 0, localpref 100, valid, external, multipath
Received Path ID 0, Local Path ID 0, version 0
Extended community: LB:10:48
Origin-AS validity: (disabled)
Path #3: Received by speaker 0
Not advertised to any peer
10
12.12.12.4 from 12.12.12.4 (192.168.0.4)
Origin incomplete, metric 0, localpref 100, valid, external, multipath
Received Path ID 0, Local Path ID 0, version 0
Extended community: LB:10:48
Origin-AS validity: (disabled)

22:35 30-09-2021

BGP provides a mechanism, known as Message Digest 5 (MD5) authentication, for authenticating a TCP segment between two BGP peers by using a clear text or encrypted password.

MD5 authentication is configured at the BGP neighbor level. BGP peers using MD5 authentication are configured with the same password. If the password authentication fails, then the packets are not transmitted along the segment.

Changing the autonomous system number is necessary when two separate BGP networks are combined under a single autonomous system. The neighbor local-as command is used to configure BGP peers to support two local autonomous system numbers to maintain peering between two separate BGP networks.

However, when the neighbor local-as command is configured on a BGP peer, the local AS number is automatically prepended to all routes that are learned from eBGP peers by default. This behavior, however, makes changing the autonomous system number for a service provider or large BGP network difficult, because the routes with the prepended AS number are rejected by internal BGP (iBGP) peers that belong to the same AS.

Hiding the local AS number by using the no-prepend command simplifies the process of changing the autonomous system number in a Border Gateway Protocol (BGP) network. Without this feature, internal BGP (iBGP) peers reject external routes from peers with a local AS number in the as-path attribute to prevent routing loops. Hiding the local AS number allows you to transparently change the autonomous system number for the entire BGP network and ensure that routes can be propagated throughout the autonomous system, while the AS number transition is incomplete.

Autonomous system numbers (ASNs) are globally unique identifiers used to identify autonomous systems (ASs) and enable ASs to exchange exterior routing information between neighboring ASs. A unique ASN is allocated to each AS for use in BGP routing. ASNs are encoded as 2-byte numbers and 4-byte numbers in BGP.

RP/0/RP0/CPU0:router(config)# as-format [asdot | asplain]
RP/0/RP0/CPU0:router(config)# as-format asdot

Note	ASN change for BGP process is not currently supported via commit replace command.

One way to reduce the iBGP mesh is to divide an autonomous system into multiple sub-autonomous systems and group them into a single confederation. To the outside world, the confederation looks like a single autonomous system. Each autonomous system is fully meshed within itself and has a few connections to other autonomous systems in the same confederation. Although the peers in different autonomous systems have eBGP sessions, they exchange routing information as if they were iBGP peers. Specifically, the next hop, MED, and local preference information is preserved. This feature allows you to retain a single IGP for all of the autonomous systems.

The Border Gateway Protocol (BGP) Additional Paths feature modifies the BGP protocol machinery for a BGP speaker to be able to send multiple paths for a prefix. This gives 'path diversity' in the network. The add path enables BGP prefix independent convergence (PIC) at the edge routers.

BGP add path enables add path advertisement in an iBGP network and advertises the following types of paths for a prefix:

• Backup paths—to enable fast convergence and connectivity restoration.

• Group-best paths—to resolve route oscillation.

• All paths—to emulate an iBGP full-mesh.

The 128-Multipath ECMP feature enables the router to support up to 128 parallel multipaths to a destination. BGP multipath functinonality enables you to create multiple BGP paths, both internal and external, to the FIB and RIB. The availablity of multiple BGP paths allows BGP to load-balance traffic over multiple links.

Equal-cost multi-path routing (ECMP) is a routing strategy where next-hop packet forwarding to a single destination can occur over multiple “best paths” which tie for top place in routing metric calculations. Multipath routing can be used in conjunction with most routing protocols, since it is a per-hop decision that is limited to a single router. It potentially offers substantial increases in bandwidth by load-balancing traffic over multiple paths. 128-path ECMP feature enables the router to support up to 128 parallel paths to a destination.

NCS 5500 Series Routers supports configuration of up to 128 ECMP next hops for BGP in IPv4 and IPv6. Support for a maximum of 128 path ECMP is available in the global table for BGP, particularly in iBGP and eBGP prefixes. High Availability supports 128 ECMPs where all prefixes and paths are synchronised to a standby RIB.

Configure 128-Multipath BGP ECMP

• Configure 128-Multipath ECMP in iBGP

• Configure 128-Multipath ECMP in eBGP

• Configure 128-Multipath ECMP in eiBGP

Note	You can either configure eBGP multipath or iBGP multipath, or both together. However, if eiBGP multipath configuration is already present, you cannot configure iBGP multipath or eBGP multipath. You have to unconfigure eiBGP mutipath configuration to configure iBGP or eBGP multipath configuration.

Router(config)#router bgp 100
Router(config-bgp)#bgp router-id 10.10.10.11
Router(config-bgp)#address-family ipv4 unicast
Router(config-bgp-af)#maximum-paths ibgp 128
Router(config-bgp-af)#exit
Router(config-bgp)#address-family ipv6 unicast
Router(config-bgp-af)#maximum-paths ibgp 128
Router(config-bgp-af)#commit

Router(config)#router bgp 100
Router(config-bgp)#bgp router-id 10.10.10.11
Router(config-bgp)#address-family ipv4 unicast
Router(config-bgp-af)#maximum-paths ebgp 128
Router(config-bgp-af)#exit
Router(config-bgp)#address-family ipv6 unicast
Router(config-bgp-af)#maximum-paths ebgp 128
Router(config-bgp-af)#commit

Router(config)#router bgp 100
Router(config-bgp)#bgp router-id 10.10.10.11
Router(config-bgp)#address-family ipv4 unicast
Router(config-bgp-af)#maximum-paths eibgp 128
Router(config-bgp-af)#exit
Router(config-bgp)#address-family ipv6 unicast
Router(config-bgp-af)#maximum-paths eibgp 128
Router(config-bgp-af)#commit

Router# show run router bgp
router bgp 100
  bgp router-id 10.10.10.11
  address-family ipv4 unicast
   maximum-paths ibgp 128
 !
 address-family ipv6 unicast
  maximum-paths ibgp 128

router bgp 100
  bgp router-id 10.10.10.11
  address-family ipv4 unicast
   maximum-paths ebgp 128
 !
 address-family ipv6 unicast
  maximum-paths ebgp 128

router bgp 100
  bgp router-id 10.10.10.11
  address-family ipv4 unicast
   maximum-paths eibgp 128
 !
 address-family ipv6 unicast
  maximum-paths eibgp 128

Router# show bgp 192.0.2.254/24
Fri Aug 14 13:59:45.190 UTC
BGP routing table entry for 191.1.0.0/24
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker              11008       11008
Last Modified: Aug 14 13:59:39.403 for 00:00:05
Paths: (35 available, best #1)
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    100.101.3.2     
  Path #1: Received by speaker 0
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    100.101.3.2     
  Local, (received & used)
    120.0.101.1 from 120.0.101.1 (120.0.101.1)
      Origin IGP, localpref 100, valid, internal, best, group-best, multipath
      Received Path ID 0, Local Path ID 1, version 7708
  Path #2: Received by speaker 0
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    100.101.3.2     
  Local, (received & used)
    120.0.102.1 from 120.0.102.1 (120.0.102.1)
      Origin IGP, localpref 100, valid, internal, multipath
      Received Path ID 0, Local Path ID 6, version 11008
........
Path #128: Received by speaker 0
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    100.101.3.2     
  Local, (received & used)
    120.0.227.1 from 120.0.227.1 (120.0.227.1)
      Origin IGP, localpref 100, valid, internal, multipath
      Received Path ID 0, Local Path ID 6, version 14008

Router# show cef 192.0.2.254/24
Fri Aug 14 14:09:50.987 UTC
191.1.0.0/24, version 46115, internal 0x5000001 0x40 (ptr 0xd236928) [1], 0x0 (0xe715668), 0x0 (0x0)
 Updated Aug 14 13:59:39.007
 Prefix Len 24, traffic index 0, precedence n/a, priority 4
   via 120.0.101.1/32, 3 dependencies, recursive, bgp-multipath [flags 0x6080]
    path-idx 0 NHID 0x0 [0xd236a00 0x0]
    next hop 120.0.101.1/32 via 120.0.101.1/32
   via 120.0.102.1/32, 3 dependencies, recursive, bgp-multipath [flags 0x6080]
    path-idx 1 NHID 0x0 [0xde9a6d0 0x0]
    next hop 120.0.102.1/32 via 120.0.102.1/32
   via 120.0.103.1/32, 3 dependencies, recursive, bgp-multipath [flags 0x6080]
    path-idx 2 NHID 0x0 [0xde9a010 0x0]
   ….
 via 120.0.227.1/32, 3 dependencies, recursive, bgp-multipath [flags 0x6080]
    path-idx 127 NHID 0x0 [0xde9a010 0x0]

Routing# show route 192.0.2.254/24
Fri Aug 14 14:11:27.403 UTC

Routing entry for 191.1.0.0/24
  Known via "bgp 1", distance 200, metric 0, type internal
  Installed Aug 14 13:59:38.971 for 00:11:48
  Routing Descriptor Blocks
    120.0.101.1, from 120.0.101.1, BGP multi path
      Route metric is 0
    120.0.102.1, from 120.0.102.1, BGP multi path
      Route metric is 0
    …
    120.0.227.1, from 120.0.227.1, BGP multi path
      Route metric is 0

Router# show bgp ipv4 unicast 191.1.0.0/24 | i multipath | utility wc -l 
Tue Jun 30 12:40:52.432 UTC 
128

Router# show bgp ipv6 unicast 191:1::/64 | i multipath | utility wc -l 
Tue Jun 30 12:42:28.893 UTC 
128

Router# show route ipv4 191.1.0.0/24 | i multipath | utility wc -l 
Tue Jun 30 12:40:53.253 UTC 
128 
Router# show cef ipv4 191.1.0.0/24 | i multipath | utility wc -l 
Tue Jun 30 12:40:53.864 UTC 
128

Router# show route ipv6 191:1::/64 | i multi | utility wc -l 
Tue Jun 30 12:42:29.709 UTC 
128
Router# show cef ipv6 191:1::/64 | i multipath | utility wc -l 
Tue Jun 30 12:42:30.332 UTC 
128

Router# show cef misc 
Platform capabilities:
--------------------------
L3 loadbalancing levels: 2
L3 Hash buckets: 64
L3 recursive Hash buckets: 128
L3 Unequal cost hash buckets: 64

The maximum-prefix feature imposes a maximum limit on the number of prefixes that are received from a neighbor for a given address family. Whenever the number of prefixes received exceeds the maximum number configured, the BGP session is terminated, which is the default behavior, after sending a cease notification to the neighbor. The session is down until a manual clear is performed by the user. The session can be resumed by using the clear bgp command. It is possible to configure a period after which the session can be automatically brought up by using the maximum-prefix command with the restart keyword. The maximum prefix limit can be configured by the user.

Note	Starting IOS-XR Release 7.3.1, the router does not apply default limits if the user does not configure the maximum number of prefixes for the address family.

Discard Extra Paths

The benefits of discard extra paths option are:

• Limits the memory footstamp of BGP.

• Stops the flapping of the peer if the paths exceed the set limit.

When the discard extra paths configuration is removed, BGP sends a route-refresh message to the neighbor if it supports the refresh capability; otherwise the session is flapped.

On the same lines, the following describes the actions when the maximum prefix value is changed:

• If the maximum value alone is changed, a route-refresh message is sourced, if applicable.

• If the new maximum value is greater than the current prefix count state, the new prefix states are saved.

• If the new maximum value is less than the current prefix count state, then some existing prefixes are deleted to match the new configured state value.

There is currently no way to control which prefixes are deleted.

The best–external path functionality supports advertisement of the best–external path to the iBGP and Route Reflector peers when a locally selected bestpath is from an internal peer. BGP selects one best path and one backup path to every destination. By default, selects one best path . Additionally, BGP selects another bestpath from among the remaining external paths for a prefix. Only a single path is chosen as the best–external path and is sent to other PEs as the backup path. BGP calculates the best–external path only when the best path is an iBGP path. If the best path is an eBGP path, then best–external path calculation is not required.

The procedure to determine the best–external path is as follows:

1. Determine the best path from the entire set of paths available for a prefix.

2. Eliminate the current best path.

3. Eliminate all the internal paths for the prefix.

4. From the remaining paths, eliminate all the paths that have the same next hop as that of the current best path.

5. Rerun the best path algorithm on the remaining set of paths to determine the best–external path.

BGP considers the external and confederations BGP paths for a prefix to calculate the best–external path. BGP advertises the best path and the best–external path as follows:

• On the primary PE—advertises the best path for a prefix to both its internal and external peers

• On the backup PE—advertises the best path selected for a prefix to the external peers and advertises the best–external path selected for that prefix to the internal peers

When a primary PE-CE link fails, BGP withdraws the route corresponding to the primary path along with its local label and programs the backup path in the Routing Information Base (RIB) and the Forwarding Information Base (FIB), by default.

However, until all the internal peers of the primary PE reconverge to use the backup path as the new bestpath, the traffic continues to be forwarded to the primary PE with the local label that was allocated for the primary path. Hence the previously allocated local label for the primary path must be retained on the primary PE for some configurable time after the reconvergence. BGP Local Label Retention feature enables the retention of the local label for a specified period. If no time is specified, the local lable is retained for a default value of five minutes.

When a Border Gateway Protocol (BGP) speaking router that has no local policy configured, receives multiple network layer reachability information (NLRI) from the internal BGP (iBGP) for the same destination, the router will choose one iBGP path as the best path. The best path is then installed in the IP routing table of the router. The iBGP Multipath Load Sharing feature enables the BGP speaking router to select multiple iBGP paths as the best paths to a destination. The best paths or multipaths are then installed in the IP routing table of the router.

iBGP Multipath Load Sharing Reference provides additional details.

Route dampening is a BGP feature that minimizes the propagation of flapping routes across an internetwork. A route is considered to be flapping when it is repeatedly available, then unavailable, then available, then unavailable, and so on.

For example, consider a network with three BGP autonomous systems: autonomous system 1, autonomous system 2, and autonomous system 3. Suppose the route to network A in autonomous system 1 flaps (it becomes unavailable). Under circumstances without route dampening, the eBGP neighbor of autonomous system 1 to autonomous system 2 sends a withdraw message to autonomous system 2. The border router in autonomous system 2, in turn, propagates the withdrawal message to autonomous system 3. When the route to network A reappears, autonomous system 1 sends an advertisement message to autonomous system 2, which sends it to autonomous system 3. If the route to network A repeatedly becomes unavailable, then available, many withdrawal and advertisement messages are sent. Route flapping is a problem in an internetwork connected to the Internet, because a route flap in the Internet backbone usually involves many routes.

The route dampening feature minimizes the flapping problem as follows. Suppose again that the route to network A flaps. The router in autonomous system 2 (in which route dampening is enabled) assigns network A a penalty of 1000 and moves it to history state. The router in autonomous system 2 continues to advertise the status of the route to neighbors. The penalties are cumulative. When the route flaps so often that the penalty exceeds a configurable suppression limit, the router stops advertising the route to network A, regardless of how many times it flaps. Thus, the route is dampened.

The penalty placed on network A is decayed until the reuse limit is reached, upon which the route is once again advertised. At half of the reuse limit, the dampening information for the route to network A is removed.

Note	No penalty is applied to a BGP peer reset when route dampening is enabled, even though the reset withdraws the route.

External BGP (eBGP) neighbors must have an inbound and outbound policy configured. If no policy is configured, no routes are accepted from the neighbor, nor are any routes advertised to it. This added security measure ensures that routes cannot accidentally be accepted or advertised in the case of a configuration omission error.

Note	This enforcement affects only eBGP neighbors (neighbors in a different autonomous system than this router). For internal BGP (iBGP) neighbors (neighbors in the same autonomous system), all routes are accepted or advertised if there is no policy.

Remotely triggered black hole (RTBH) filtering is a technique that provides the ability to drop undesirable traffic before it enters a protected network. RTBH filtering provides a method for quickly dropping undesirable traffic at the edge of the network, based on either source addresses or destination addresses by forwarding it to a null0 interface. RTBH filtering based on a destination address is commonly known as Destination-based RTBH filtering. Whereas, RTBH filtering based on a source address is known as Source-based RTBH filtering.

RTBH filtering is one of the many techniques in the security toolkit that can be used together to enhance network security in the following ways:

• Effectively mitigate DDoS and worm attacks

• Quarantine all traffic destined for the target under attack

• Enforce blocklist filtering

Note	RTBH is not supported in cases such as L3VPN iBGP route over NULL0.

Note	On NCS 5700 line cards with NPU-based platforms, when you configure a NULL0 route, both destination-based RTBH filtering (D-RTBH) and source-based RTBH filtering (S-RTBH) are enforced. This behavior is irrespective of Unicast Reverse Path Forwarding (URPF) interface configuration.

Note	URPF infrastructure can be disabled globally by using the hw-module fib urpf disable command.

BGP requires that all iBGP speakers be fully meshed. However, this requirement does not scale well when there are many iBGP speakers. Instead of configuring a confederation, you can reduce the iBGP mesh by using a route reflector configuration. With route reflectors, all iBGP speakers need not be fully meshed because there is a method to pass learned routes to neighbors. In this model, an iBGP peer is configured to be a route reflector responsible for passing iBGP learned routes to a set of iBGP neighbors.

In #concept_3069A67F47124831B884817915F11DF3__ , Router B is configured as a route reflector. When the route reflector receives routes advertised from Router A, it advertises them to Router C, and vice versa. This scheme eliminates the need for the iBGP session between routers A and C.

See BGP Route Reflectors Reference for additional details on route reflectors.