With OSPF area 0 fully configured and authenticated between the PE routers and core, this video extends the topology by adding the CE1 to PE1 link in OSPF area 1 — introducing inter-area routing and making PE1 an area border router. We verify that type 3 summary LSAs are generated correctly and that inter-area routes are propagating as expected.

CE1 is a new logical system added to the topology in this video. Its loopback address is 11.11.11.11/32, its router ID is set to match, and its logical tunnel interface towards PE1 uses the 172.16.1.0/24 subnet. The interface is LT000 unit 111 on the CE1 side, peering with unit 11 on the PE1 side — the three-digit unit numbering reflects that this is an external link using a different addressing convention to the core PE mesh.

OSPF area 1 is configured on the CE1 to PE1 link. On PE1, the loopback is kept as passive in area 0 and the new LT interface is added to area 1, making PE1 an ABR. On CE1 all interfaces including the loopback go into area 1. Once committed the OSPF adjacency forms and we verify it is in Full state using show ospf neighbor on CE1.

The routing table on CE1 after the adjacency comes up shows the full area 0 topology — loopbacks and links from all four PE routers and the core — all appearing as inter-area OSPF routes. The administrative distance for inter-area and intra-area routes is the same in Junos — both show a preference of 10 — which is worth noting if you’re comparing behaviour to other platforms.

Examining the OSPF database on PE1 after it becomes an ABR shows all three LSA types — type 1 router LSAs, type 2 network LSAs, and type 3 summary LSAs. The type 3 LSAs are what PE1 generates to advertise area 0 routes into area 1 and vice versa. Seeing all three LSA types in the database confirms ABR functionality is working correctly.