AS-Path Filtering: Accept Only Neighbor’s Own Routes

Bundle lab

Advanced CCNP BGP policy lab focusing on inbound AS-path filtering at the edge. You will build a simple eBGP chain (AS 65003 — AS 65002 — AS 65001), originate prefixes from the two upstream routers, confirm that the edge learns multiple routes, then enforce a policy on the edge (R1) to accept only routes originated by its directly connected eBGP neighbor (AS 65002) while rejecting routes that transited AS 65003. Verification relies entirely on R1’s BGP table and AS-path regular-expression queries.

Learning objectives

• Configure eBGP adjacencies over /30 point-to-point transits with correct remote-as values
• Originate exact-match prefixes into BGP and understand network mask requirements
• Implement inbound AS-path filtering using Cisco regex and filter-lists
• Verify routes and AS-paths using show ip bgp and show ip bgp regexp
• Reason about policy effects on accepted and rejected routes

Troubleshooting focus

• Diagnose eBGP session formation issues (wrong remote-as or address reachability)
• Identify missing prefix origination due to absent exact RIB match or mask keyword
• Confirm filter direction and sequence so that intended routes are not unintentionally dropped
• Use regex testing with show ip bgp regexp to confirm matching behavior before committing policy