BGP-received-route Tracking and Fast Peering Deactivation:

By: Sajid Ali Pathan

This is quite new think that we have ever done with BGP. In a BGP dual multi-home scenario we configured "Selective Route Tracking" with "neighbor fall-over" command under BGP process. Actually we were facing some kind of unusual issue during internet outage from primary ISP. We lost internet connectivity but local peering with ISP remains up.

“When you configured Neighbor fall-over command under BGP process, it monitors RIB and if route to peer is not present in routing table it will immediately deactivate peer session without waiting for hold down timer.”

First thing first, Configured an object that track BGP-received-route, in our case this was a route that ISP receives from upstream provider and advertise to downstream (Remember that I didn't track default route).

RTR-1(config)# track 10 ip route 221.120.x.x 255.255.255.255 reachability

RTR-1#sh track 10

Track 10

  IP route 221.120.x.x 255.255.255.255 reachability

  Reachability is Up (BGP)

    2 changes, last change 00:00:16

  Delay up 2 secs, down 2 secs

  First-hop interface is FastEthernet3/0

  Tracked by:

    STATIC-IP-ROUTING 0

Next, create a host route for the BGP next hop pointing to the next hop itself.

RTR-1(config)# ip route 192.168.x.113 255.255.255.255 FastEthernet3/0 track 10

Above mentioned route will remove from routing table when track 10 returns false value.

Current output of “sh ip route” command:

      192.168.x.0/24 is variably subnetted, 3 subnets, 2 masks

C        192.168.x.112/30 is directly connected, FastEthernet3/0

S        192.168.x.113/32 is directly connected, FastEthernet3/0

L        192.168.x.114/32 is directly connected, FastEthernet3/0

Now, configure prefix-list matches exactly neighbor IP Address and then apply it in route-map and attached in "neighbor fall-over" command.

RTR-1(config)# ip prefix-list LPB seq 5 permit 192.168.x.113/32

!

RTR-1(config)# route-map Edge_R permit 10

RTR-1(config -route-map)# match ip address prefix-list LPB

!

RTR-1(config)# router bgp 12345

RTR-1(config-router)# neighbor 192.168.x.113 fall-over route-map Edge_R

Below logs appears on console when BGP receive route remove from BGP RIB:

*May  9 10:55:18.643: %TRACKING-5-STATE: 10 ip route 221.120.x.x/32 reachability Up->Down

*May  9 10:55:18.683: %BGP-5-ADJCHANGE: neighbor 192.168.x.113 Down Route to peer lost

*May  9 10:55:18.683: %BGP_SESSION-5-ADJCHANGE: neighbor 192.168.x.113 IPv4 Unicast topology base removed from session  Route to peer lost

Now look, “sh ip route” command output:

      192.168.x.0/24 is variably subnetted, 2 subnets, 2 masks

C        192.168.x.112/30 is directly connected, FastEthernet3/0

L        192.168.x.114/32 is directly connected, FastEthernet3/0

Static route to peer removed from routing table.