Link-local neighbors and router IDs
OSPFv3 was defined for IPv6. It runs on a link and ordinarily exchanges packets using IPv6 link-local source addresses and next header 89. A router still uses a 32-bit router ID, commonly displayed in dotted decimal.
Matching area and instance IDs let routers participate in the same OSPFv3 instance on a link. Each endpoint needs its IPv6 link-local address, while a global IPv6 subnet on the transit link is optional. OSPFv3 retains areas, the backbone and the cost-based SPF calculation. The address-family extension also lets OSPFv3 advertise IPv4 routes.
Separate topology from IPv6 prefixes
Router LSAs and network LSAs describe the graph inside an area. Intra-area-prefix LSAs associate IPv6 prefixes with a router or transit network in that graph. The topology and prefix advertisements can therefore change separately.
A Link-LSA has link-local flooding scope and provides information such as a router’s link-local address. An intra-area-prefix LSA can carry a prefix across the area so other routers can install a route to it.
Try it in a lab
- Use an OSPFv3-capable pair of routers in area 0. Give the point-to-point transit interfaces only link-local IPv6 addresses, and advertise a global IPv6 prefix from a separate interface.
- Verify that the transit neighbor reaches Full. On the receiving router, find the advertised prefix, its link-local next hop and the transit interface used to reach that next hop.
- Withdraw the advertised prefix while keeping the transit link configured. Verify that the prefix disappears from the routing table while the neighbor remains Full.