BGP Tables

Question 1

BGP Tables

Answer 1

BGP uses three tables for maintaining a network prefix and the path attributes (PA)
associated with each path for a prefix.

1. Adj-RIB-in - show ip bgp neighbor x.x.x.x received-routes
2. Loc-RIB - show ip bgp
3. Adj-RIB-out - show ip bgp neighbor x.x.x.x advertised-routes

Question 2

Adj-RIB-in

Answer 2

Contains the Network Layer Reachability Information (NLRI) (network
prefix and prefix-length) and BGP PAs in the original form that they are received
from a BGP peer.
The Adj-RIB-in table is purged after all route policies are processed to save memory.

Question 3

Loc-RIB

Answer 3

Contains all the NLRIs that originate locally or that came from the
Adj-RIB-in table after passing any inbound route policy processing. Note that BGP
PAs in the Loc-RIB might have been modified by the router’s inbound route policies
for a specific neighbor.

After NLRIs pass the validity and next-hop reachability check, the BGP best path
algorithm selects the best NLRI for a specific prefix. The Loc-RIB table is the table
used for presenting routes to the IP routing table and is the BGP table commonly
referred to as “The BGP table.”

Question 4

Adj-RIB-out

Answer 4

Contains the NLRIs after outbound route policies have processed.
Different outbound route policies could exist between neighbors, so the Adj-RIB-
out keeps track of the neighbors and the NLRIs for each neighbor.
Outbound route policies could modify PAs for a specific neighbor.

Question 5

Answer 5

Learned routes from a neighbor before any inbound route policy is applied:

show ip bgp neigbor x.x.x.x received-routes - Adj-RIB-In

BGP table:

show ip bgp - Loc-RIB

Advertised routes to neighbor after applying outbound route policy:

show ip bgp neighbor x.x.x.x advertised-routes - Adj-RIB-Out