Lab 3: iBGP over Loopbacks with OSPF Reachability

Bundle lab · $29.99

Build an internal BGP (iBGP) peering between two IOS routers in the same AS over stable Loopback0 addresses, with OSPF providing loopback reachability. Each router originates a /24 from Loopback1 into BGP, and next-hop/peering behavior is validated from end hosts. This lab emphasizes the deterministic neighbor configuration (remote-as, update-source Loopback0, router-id) and exact-match network origination, supported by a minimal, secure OSPF core.

Learning objectives

• Configure OSPF area 0 to provide stable reachability to iBGP peering loopbacks with proper router-ids and passive-interface hygiene.
• Establish an iBGP session in AS 65001 over Loopback0 with neighbor update-source Loopback0 and matching remote-as on both ends.
• Originate exact /24 networks from Loopback1 using the network ... mask command and validate advertisement reception on the peer.
• Explain why update-source Loopback0 is mandatory for iBGP peering over loopbacks and how reachability underpins BGP TCP formation.
• Verify and troubleshoot iBGP and OSPF control-plane formation using show commands, and validate data-plane reachability from end hosts.

Troubleshooting focus

• If the iBGP neighbor stays Idle/Active, verify IP reachability to the remote Loopback0 (OSPF learning the /32) and ensure both sides use update-source Loopback0.
• If the iBGP session forms but the /24s do not appear in the BGP table, confirm the exact network ... mask statement matches the installed route for the loopback /24.
• If host-to-remote loopback pings fail, check that OSPF carries the local user LAN as passive so the remote router has a return path.
• If OSPF adjacency does not form, validate area 0 on both sides, the correct wildcard mask for the /30 transit, and that the transit interface is not passive.
• If paths appear but next-hop is unreachable, ensure the peer statements are iBGP (same AS) and consider next-hop-self where appropriate.