BGP Troubleshooting Capstone: eBGP + iBGP Repair

Bundle lab

Advanced BGP capstone, run as a fault hunt. The hub lost its upstream service route overnight and the branch cannot see hub prefixes either. Sessions are not all down, which is what makes it interesting: one peering never establishes, one establishes and carries nothing, and one prefix is never originated at all. You get symptoms and the required end state — no fault list. Faults span all three routers, and one of them is not in BGP at all.

Learning objectives

• Diagnose BGP from operational state alone, with no list of what was changed
• Separate a session that will not establish from a session that is up and carrying no prefixes
• Recognise that an iBGP peering depends on the IGP underneath it, and check the underlay before the overlay
• Know why a network statement whose mask does not match the interface prefix originates nothing
• Repair faults across three routers rather than assuming the edge router owns the problem

Troubleshooting focus

• Use show ip bgp summary to confirm BGP neighbor state and timers
• Compare configured remote-as versus the actual neighbor ASN
• Verify iBGP loopback reachability with OSPF and update-source Loopback0 on both ends
• Inspect received routes and next-hop addresses on downstream iBGP peers
• Validate data-plane from the end hosts with ping and traceroute once control-plane is corrected