LAB: BGP Fast Fallover

- Disable the BGP feature in R3 that allows for eBGP peering session deactivation when a physical interface goes down.
- Configure all R3’s BGP peering session for fast peering deactivation.

It is common to have eBGP peers directly connected across a physical interface.  With point-to-point links this allows for fast external session deactivation based on the interface protocol state. That is, as soon as the interface connecting to an external peer signals protocols down, the BGP process may deactivate the peering session without waiting for the hold-down time to expire. This feature is enabled by default for eBGP sessions using the BGP process command 

bgp fast-external-fallover

Notice that this feature is only efficient when the peering session is established across the non-shared link – using it on NBMA and Ethernet interfaces might be inefficient.

The new feature is called “fast peering session deactivation support” and could be
configured on a per-neighbor basis using the command 

neighbor <IP> fall-over 

This feature applies to both iBGP and eBGP (mostly multi-hop) sessions and does not depend on interface state. The IGP route used to reach the peer configured for fast session deactivation is monitored by the BGP process. Once the IGP route disappears, the BGP session gets immediately torn down, unless there is a backup IGP route to reach the peer. Thus, the new feature is event driven and allows for fast detection of peering issues even for iBGP (non-direct) sessions. Notice that this feature has the same limitation as the fast-external-fallover – if the peer is connected via a shared interface, the router may not detect the loss of the directly connected IGP network. In situations like this, you may use point-to-point NBMA subinterfaces or reliable static /32 routes on the shared NBMA or Ethernet interfaces.  Keep in mind that convergence improvements result in less stable topology. In order to minimize the impact of IGP instabilities on BGP tables, use event dampening technologies (such as ip dampening) and prefix summarization.

configuration:

R3:
router bgp 200
no bgp fast-external-fallover
neighbor 155.1.0.5 fall-over
neighbor 155.1.13.1 fall-over
neighbor 155.1.23.2 fall-over
neighbor 155.1.37.7 fall-over
neighbor 155.1.58.8 fall-over

neighbor 155.1.108.10 fall-over