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A Look at OSPF Routing on the SSR
The Open Shortest Path Routing (OSPF) protocol is a link-state protocol. It is an Interior
Gateway Protocol (IGP) that distributes routing information between routers in a single
autonomous system.
OSPF chooses the least cost path as the best path. In a link-state protocol, each router
maintains a database describing the entire autonomous-system topology, which it builds
out of the collected link-state advertisements of all routers. Each participating router
distributes its local state (such as the router's usable interfaces and reachable neighbors)
throughout the autonomous system by flooding.
Each multiaccess network that has at least two attached routers has a designated router
and a backup designated router. The designated router floods a link state advertisement
for the multiaccess network and has other special responsibilities. The designated router
concept reduces the number of adjacencies required on a multiaccess network. An
adjacency is an OSPF relationship formed between selected neighboring routers for the
purpose of exchanging routing information. Not every pair of neighboring routers
become adjacent.
OSPF allows networks to be grouped into areas. Routing information passed between
areas is abstracted, potentially allowing a significant reduction in routing traffic. OSPF
uses four different types of routes, listed in order of preference: intra-area, inter-area, type
1 external, and type 2 external. Intra-area paths have destinations within the same area,
inter-area paths have destinations in other OSPF areas and Autonomous System External
(ASE) routes are routes to destinations external to the autonomous system.
Routes imported into OSPF as type 1 routes are supposed to be from IGPs whose external
metrics are directly comparable to OSPF metrics. When a routing decision is being made,
CoreWatch User's Manual
Configuring OSPF
Chapter 14
on the SSR
247
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