34 OSPF C ONFIGURATIONOSPF OverviewIntroduction to OSPF Open shortest path first (OSPF) is a link state-based interior gateway protocoldeveloped by IETF. At present, OSPF version 2 (RFC 2328) is used, which has thefollowing features:■ High applicability: OSPF supports networks of various sizes and can support upto several hundred routers.■ Fast convergence: OSPF can transmit update packets immediately after thenetwork topology changes so that the change can be synchronized in theautonomous system (AS).■ Loop-free: Since OSPF calculates routes with the shortest path tree algorithmaccording to the collected link states, it guarantees that no loop routes will begenerated from the algorithm basis.■ Area partition: OSPF allows an autonomous system network to be divided intodifferent areas for convenient management so that routing informationtransmitted between the areas is abstracted further, thereby reducing networkbandwidth consumption.■ Equivalent route: OSPF supports multiple equivalent routes to the samedestination.■ Routing hierarchy: OSPF has a four-level routing hierarchy. It prioritizes theroutes as intra-area, inter-area, external type-1, and external type-2 routes.■ Authentication: OSPF supports interface-based packet authentication toguarantee the security of route calculation.■ Multicast transmission: OSPF supports transmitting protocol packets inmulticast mode.OSPF Route Calculation Taking no account of area partition, the routing calculation process of the OSPFprotocol is as follows:■ Each OSPF-capable router maintains a link state database (LSDB), whichdescribes the topology of the whole AS. According to the network topologyaround itself, each router generates a link state advertisement (LSA). Routerson the network exchange LSAs with each other by transmitting protocolpackets. Thus, each router receives the LSAs of other routers and all these LSAsform the LSDB of the router.■ An LSA describes the network topology around a router, whereas an LSDBdescribes the network topology of the whole network. Routers can easilytransform the LSDB to a weighted directed map, which actually reflects the
