2.1.4 Link-state routing algorithms maintain a complex database of the network topology by exchanging link-state advertisements (LSAs) with other routers in a network. This page describes the link-state routing algorithm.
Link-state routing algorithms have the following characteristics:
- They are known
collectively as SPF protocols.
- They maintain a
complex database of the network topology.
- They are based
on the Dijkstra algorithm.
Each router constructs a topological database from the LSAs that it receives. The SPF algorithm is then used to compute the reachability of destinations. This information is used to update the routing table. This process can discover changes in the network topology caused by component failure or network growth.
An LSA exchange is triggered by an event in the network instead of periodic updates. This speeds up the convergence process because there is no need to wait for a series of timers to expire before the routers can converge. If the network shown in Figure
uses a link-state routing protocol, there is
no concern about connectivity between routers A and D. Based on the protocol
that is employed and the metrics that are selected, the routing protocol can
discriminate between two paths to the same destination and use the best one. In
Figure 
there are two routing entries in the table
for the route from Router A to Router D. In this figure, the routes have equal
costs so the link-state routing protocol records both routes. Some link-state
protocols provide a way to assess the performance capabilities of the two
routes and choose the best one. If the preferred route through Router C
experiences operational difficulties such as congestion or component failure,
the link-state routing protocol can detect this change and route packets
through Router B.The next page will describe some advantages of link-state protocols.
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