BGP Route Advertisement

In the inter-site redundancy deployment, the Multi-exit Discriminator (MED) attribute, which is a non-transitive BGP attribute, is used to inform the neighboring routers of the best path to reach the active service VIP. However, MED values are not shared beyond neighboring routers to their eBGP peers.

In the inter-rack deployment model, AS-path prepending technique is used in BGP to influence the path selection of routes advertised to other BGP routers. AS-path prepending technique makes a certain route more favorable over another by manipulating the AS-path in the BGP route advertisement. The difference in the number of AS-paths getting prepended by software logic helps to provide guidance to upstream routers to choose the best routable path.

Following table captures the number of AS-path prepend required for various combination of service VIP and interface bonding status.

GR Role Bonding interface active VIP present MED value

Local Preference

AS-Path Prepend (Number of times)

Primary Role (active) Yes Yes 1210 2220 3
Yes No 1220 2210 3
No Yes 1215 2215 3
No No 1225 2205 3
Standby Role Yes Yes 2210 1220 6
Yes No 2220 1210 6
No Yes 2215 1215 6
No No 2225 1205 6
Standby-Error Role NA NA 3220 220 9