Troubleshooting guide

Additional BGP Configuration

Distribute List

Use the distribute-list command to add route filtering functionality by assigning inbound and outbound

IPv4 access control lists (ACLs) to a BGP neighbor. Only one inbound/outbound pair of ACLs can be

configured for a particular neighbor.

(config-bgp-neighbor)#distribute-list <ipv4 acl name> [in | out]

<ipv4 acl name> Specifies an IPv4 ACL name. This is a standard or extended IPv4 ACL against which the

contents of the incoming/outgoing routing updates are matched.

in Applies route filtering to inbound data.

out Applies route filtering to outbound data.

eBGP Multihop

Use the ebgp-multihop command to configure the maximum hop count for BGP messages to a neighbor.

(config-bgp-neighbor)#ebgp-multihop <value>

<value> Specifies the maximum hop count of BGP messages to a neighbor. Range is 1 to 254 hops.

This command allows an eBGP neighbor to be on a network that is not directly connected. The default

time to live (TTL) for BGP messages is 1 since eBGP peers are normally directly connected. However, in

certain applications, a non-BGP device, such as a firewall or router, might reside between eBGP peers. The

ebgp multihop command is required in this case to allow updates to have a TTL greater than 1 and to

allow received BGP updates to be added to the BGP table when the next-hop address is not directly

connected.

It is also good practice to create a static route to reach the eBGP neighbor when the neighbor is more than

one hop away, as well as creating a backup route pointing to null 0 interface with a higher administrative

distance. These routes prevent unnecessary BGP traffic from traversing the wrong connection and prevent

the BGP neighbor relationship from incorrectly attempting to establish itself on that connection. It is also

possible that sending invalid BGP traffic to some providers might result in the connection being

automatically disabled by the provider as a security measure.

Fast External Failover

Use the bgp fast-external-failover command to enable the fast external failover feature.

(config-bgp)#bgp fast-external-failover

When failover is enabled, if the link interface goes down between this router and a BGP neighbor, the BGP

session with the neighbor is immediately cleared. When failover is disabled and the link goes down, the

session is maintained until the BGP hold timer expires (refer to

Hold Timer on page 18).

Routes with lower administrative distances are favored.

Refer to IP ACLs in AOS (ADTRAN’s Knowledge Base article 3087) for information on

how to create a standard or extended IPv4 ACL.