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 |