Troubleshooting guide

Additional BGP Configuration

Update Source

Use the update-source command to specify which interface’s IPv4 address will be used as the source IPv4

address for the BGP TCP connection.

(config-bgp-neighbor)#update-source <interface>

<interface> Specifies the interface to be used as the source IPv4 address. Specify an interface in the

ap/radio | ap/radio.vap]>, for example, for an Ethernet interface, use eth 0/1; for a PPP interface, use

ppp

1; and for an ATM subinterface, use atm 1.1. Enter update-source ? for a complete list of valid

interfaces.

iBGP

When an AOS device sends a BGP packet, the routing table is first consulted to determine how to

reach the intended peer. By default, the IPv4 address assigned to the egress interface of the local router

the packet uses to reach the peer is the source address for the BGP packet. This address can be

overwritten by using the update source command.

Loopback interfaces should be used when there are multiple paths to reach a single iBGP neighbor.

Using a loopback address as the update source forces the BGP messages to use only the IP address of

the loopback interface instead of the IP addresses associated with the egress interface of any of the

multiple links from a single neighbor. The loopback interface is first created and then advertised as the

update source toward the intended iBGP neighbor.

eBGP

Loopback interfaces can also be used in eBGP scenarios where multiple links to the same neighbor

exist and a multilink protocol is not being used. Similar to the iBGP scenario above, the loopback

address would need to be configured and then specified with the update-source command on a

per-neighbor basis. However, some additional configuration is required for eBGP applications. Unlike

iBGP, eBGP assumes that the neighbor is one hop away on a directly connected interface. When

loopback addresses are used, an extra hop is created between the neighbors, thus requiring the use of

the ebgp-multihop command to increase the TTL value for BGP messages. If the value for this

command is not changed in accordance to the new number of hops, the TTL for BGP messages

intended for eBGP neighbors defaults to 1 and will expire before the packet reaches the destined

loopback interface on the peer router. Refer to

eBGP Multihop on page 17 for more information on this

command.

This command is most often configured as a loopback interface that is reachable by the

peer router. The peer will specify this address in its neighbor commands for this router.

Using a loopback interface as the update source for an iBGP neighbor is often done in

conjunction with an interior routing protocol, such as OSPF, that can dynamically

advertise routes to reach a particular destination when failures and topology changes

occur.

When a loopback interface is used as the update source address, one extra hop is

created between eBGP neighbors. It is important to account for this extra hop when

calculating the TTL value set in the ebgp-multihop command.