Transit AS: Carry eBGP across iBGP (next-hop-self)

Bundle lab · $29.99

In this CCNP-level lab (BGP Fundamentals Lab 4/10), you will build a small, realistic transit-AS scenario: an external route learned by R1 via eBGP from AS 65003 must be carried across iBGP to R2 inside AS 65001. You will intentionally encounter the classic iBGP next-hop problem (R2 sees an unreachable next-hop for 172.16.30.0/24) and fix it on R1 with neighbor next-hop-self. iBGP peering runs over Loopback0 addresses with reachability provided by OSPF area 0 between R1 and R2. Two hosts validate end-to-end data-plane reachability and routing control-plane state.

Learning objectives

• Stand up an eBGP peering over a /30 transit and advertise an exact-match prefix with the network ... mask command.
• Build an iBGP session over Loopback0s using update-source Loopback0 and provide reachability via OSPF area 0.
• Diagnose the iBGP next-hop problem when carrying an eBGP-learned route across iBGP inside the transit AS.
• Fix the issue with neighbor <ip> next-hop-self on the route-reflector/edge iBGP speaker so downstream iBGP peers install the route.
• Verify with show ip bgp, show ip bgp summary, and show ip route bgp; validate end-to-end reachability from hosts.

Troubleshooting focus

• If the iBGP session does not form, confirm update-source Loopback0 is configured on both sides and that OSPF provides reachability to the remote Loopback0.
• If 172.16.30.0/24 is missing on R2, check that R3 originated it with an exact-match network statement and that R1 advertises it to R2 via iBGP.
• If 172.16.30.0/24 is present in R2 BGP but not in the routing table, examine the next-hop. If it is 10.31.31.2 and unreachable, apply next-hop-self on R1.
• If pings from the hosts fail but control-plane looks correct, ensure the external neighbor has a return route to the host subnets (advertise 10.10.10.0/24 and 10.20.20.0/24 from AS 65001 to AS 65003).
• Validate OSPF area assignments and passive-interface hygiene; only the R1–R2 transit should form an OSPF adjacency.