Network Router User Manual

SmartSwitch Router User Reference Manual 111
Chapter 11
OSPF
Configuration
Guide
OSPF Overview
Open Shortest Path First Routing (OSPF) is a shortest path first or link-state protocol. The
SSR supports OSPF Version 2.0, as defined in RFC 1583. OSPF is an interior gateway
protocol that distributes routing information between routers in a single autonomous
system. OSPF chooses the least-cost path as the best path. OSPF is suitable for complex
networks with a large number of routers because it provides equal-cost multi-path routing
where packets to a single destination can be sent via more than one interface
simultaneously.
In a link-state protocol, each router maintains a database that describes the entire AS
topology, which it builds out of the collected link state advertisements of all routers. Each
participating router distributes its local state (i.e., the router's usable interfaces and
reachable neighbors) throughout the AS by flooding. Each multi-access 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 multi-access network and has
other special responsibilities. The designated router concept reduces the number of
adjacencies required on a multi-access network.
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