BGP RIB-Failure

By | January 18, 2019

When a Router receives a BGP UPDATE packet that contains Network Layer Reachability Information (NLRI) – this is, a route; the packet is processed in the next order:

– Step 1. BGP checks for the NLRI (prefix received) against any BGP inbound filter configured on the Router.

– Step 2. If the NLRI is not filtered, the prefix can be seen in the BGP table with the show ip bgp command.

– Step 3. If the Routing Table already has the same prefix/prefix-length entry with a lower Administrative Distance (AD) as seen in show ip route, BGP marks the route received with RIB-Failure.

*You can display BGP routes that are not inserted in the IP routing table with the show ip bgp rib-failure command, which also explains why the BGP route was not inserted in the IP routing table.

*all routes shown in show ip bgp rib-failure command will still advertised to all BGP peers.

*Network Layer Reachability Information (NLRI)

The Network Layer Reachability Information (NLRI) is exchanged between BGP routers using UPDATE messages. An NLRI is composed of a LENGTH and a PREFIX. The length is a network mask in CIDR notation (eg. /25) specifying the number of network bits, and the prefix is the Network address for that subnet.

The NLRI is unique to BGP version 4 and allows BGP to carry supernetting information, as well as perform aggregation.

The NLRI would look something like one of these:

     /25, 204.149.16.128
     /23, 206.134.32
     /8, 10

Reference:
1. https://blog.ipspace.net/2007/12/what-is-bgp-rib-failure.html
2. https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/213286-understand-bgp-rib-failure-and-the-bgp-s.html#anc4
3. https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/5816-bgpfaq-5816.html#anc27