CCNA: OSPFv3 Across Three Routers — Cost and Path Choice

Archive lab

Daily CCNA IPv6 lab: build OSPFv3 across a three-router triangle with dual-stack links and real end hosts. IPv4 OSPFv2 is pre-built and forwarding end-to-end as the baseline. Your tasks: bring R3 into OSPFv3 with an explicit router-id and enable it per-interface, activate OSPFv3 on R1’s R3-facing interface, then steer the preferred IPv6 path by adjusting interface cost. Verify neighbor state, routes, and end-to-end IPv6 reachability from host to host.

Learning objectives

• Form OSPFv3 adjacencies using per-interface activation and explicit router-ids
• Advertise IPv6 LANs and transit segments in OSPFv3 and verify routes
• Manipulate OSPFv3 path selection with an interface cost change
• Validate ECMP/preference and forwarding with show commands and host-based ping6/traceroute
• Contrast IPv4 (baseline OSPFv2) and IPv6 (OSPFv3) control planes on the same topology

Troubleshooting focus

• If adjacencies do not form: Confirm both ends of each transit have OSPFv3 enabled per-interface and are in the same area.; Check that IPv6 link-local addressing exists (it’s automatic) and that interfaces are up/up.
• If routes are missing: Ensure the LAN interfaces are enabled for OSPFv3 on the routers that own them.; Verify the OSPFv3 process has a valid explicit router-id.
• If the path is not the one you intend: Confirm the interface cost was set on the correct exit interface, and that it is 21 or more — a cost of exactly 20 ties with the path via R2 at 30 each, so OSPF installs both routes and nothing is steered.; Re-check after flapping the interface or waiting for SPF to run; use show ipv6 ospf interface to confirm the cost in effect.
• If host pings fail but routes look correct: Verify the host default gateways (IPv4 and IPv6), masks/prefix lengths, and that the gateway addresses match those configured on the routers.; Use traceroute -6 from the host to see where the path stops and compare with router routing tables.