BGP Lab 7: Filtering Advertised Prefixes (Prefix-List)

Bundle lab

Deploy eBGP between two routers and precisely control which locally-originated networks are advertised to a neighbor using an outbound prefix-list. R1 originates four /24 loopback routes but advertises only two to R2. Verify using IOS show commands and basic host reachability checks.

Learning objectives

• Configure an eBGP peering over a point-to-point /30 with correct remote-as values
• Originate exact-match routes into BGP using the network ... mask command
• Build and apply an outbound ip prefix-list to control advertised prefixes
• Verify advertised-routes and received routes with show ip bgp neighbors and show ip bgp
• Differentiate local BGP table visibility from eBGP neighbor visibility

Troubleshooting focus

• If the BGP session is not established: confirm both sides use the correct neighbor IP (the directly connected transit address) and matching remote-as values. Ensure the transit interface is up/up and pingable.
• If R2 receives all four routes: verify the prefix-list is applied in the correct direction (outbound on R1) and that there are no extra permits matching the suppressed prefixes.
• If R2 receives none: confirm the prefix-list exists before application and that it has the intended sequence permits. Also ensure R1 has exact routes to all four loopbacks in its RIB.
• If a specific loopback is missing from R1’s local BGP table: check the exact-match network statement and that the loopback interface is up with the correct mask.