Allied Telesis AT-8550 - Manual (Page 159)

AT-9108, AT-8518, AT-8525, and AT-8550 User’s Guide
9-5
Overview of OSPF
OSPF is a link-state protocol that distributes routing information
between routers belonging to a single IP domain, also known as an
autonomous system (AS). In a link-state routing protocol, each router
maintains a database describing the topology of the autonomous
system. Each participating router has an identical database
maintained from the perspective of that router.
From the link-state database (LSDB), each router constructs a tree of
shortest paths, using itself as the root. The shortest path tree
provides the route to each destination in the autonomous system.
When several equal-cost routes to a destination exist, traffic can
distributed among them. The cost of a route is described by a single
metric.
Link-State
Database
Upon initialization, each router transmits a link-state advertisement
(LSA) on each of its interfaces. LSAs are collected by each router and
entered into the LSDB of each router. OSPF uses flooding to
distribute LSAs between routers. Any change in routing information
is sent to all of the routers in the network. All routers within an area
have the exact same LSDB.
Table 9-1 describes LSA type numbers.
Table 9-1 LSA Type Numbers
Type Number Description
1 Router link
2 Network link
3 Summary link
4 AS summary link
5 AS external link
7 NSSA external link