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4.3. The Routing Table Structure

The routing table used by OSPF for IPv4 is defined in Section 11 of [OSPFV2]. For IPv6, there are analogous routing table entries: there are routing table entries for IPv6 address prefixes and also for AS boundary routers. The latter routing table entries are only used to hold intermediate results during the routing table build process (see Section 4.8).

Also, to hold the intermediate results during the shortest-path calculation for each area, there is a separate routing table for each area holding the following entries:

  • An entry for each router in the area. Routers are identified by their OSPF Router ID. These routing table entries hold the set of shortest paths through a given area to a given router, which in turn allows calculation of paths to the IPv6 prefixes advertised by that router in intra-area-prefix-LSAs. If the router is also an area border router, these entries are also used to calculate paths for inter-area address prefixes. If in addition the router is the other endpoint of a virtual link, the routing table entry describes the cost and viability of the virtual link.

  • An entry for each transit link in the area. Transit links have associated network-LSAs. Both the transit link and the network-LSA are identified by a combination of the Designated Router's Interface ID on the link and the Designated Router's OSPF Router ID. These routing table entries allow later calculation of paths to IP prefixes advertised for the transit link in intra-area-prefix-LSAs.

The fields in the IPv4 OSPF routing table (see Section 11 of [OSPFV2]) remain valid for IPv6: optional capabilities (routers only), path type, cost, type 2 cost, link state origin, and for each of the equal cost paths to the destination, the next-hop and advertising routers.

For IPv6, the link-state origin field in the routing table entry is the router-LSA or network-LSA that has directly or indirectly produced the routing table entry. For example, if the routing table entry describes a route to an IPv6 prefix, the link state origin is the router-LSA or network-LSA that is listed in the body of the intra-area-prefix-LSA that has produced the route (see Appendix A.4.10).

4.3.1. Routing Table Lookup​

Routing table lookup (i.e., determining the best matching routing table entry during IP forwarding) is the same for IPv6 as for IPv4.

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