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: