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IntermediatePublished 2026-07-03

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

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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.

Track
Est. time
55 minutes
Difficulty
Intermediate
Topics

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.

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How this lab is graded
  • Build it your way. Where a lab lets you choose a value — a VLAN name, an interface description — grading checks that you configured it, not which name you picked. Names that another line has to reference, like an ACL applied with access-class, are stated in the guide and do have to match.
  • Addresses, modes and protocol keywords are exact. An IP address, a subnet mask, switchport mode trunk, an encapsulation — these carry the meaning of the lab, so they are graded as written in the guide.
  • Grading reads your saved configuration. Export the lab from CML after you have configured it, and make sure anything you set is in the running-config — a change that only exists in a terminal session never reaches the grader.
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