Understanding Link State Information

Exchange 2003, like Exchange 2000, uses link state information to determine the most effective route for delivering messages. The link state table contains information about the routing topology and whether each connector in the topology is available or unavailable. Additionally, the link state table contains costs associated with each available connector. Exchange uses this information to determine the route with the lowest cost. If a connector along the lowest cost route is unavailable, Exchange determines the best alternative route, based on cost and connector availability.

To understand how link state information and connector costs work, consider the routing topology shown in the following figure, in which four routing groups exist: Seattle, Brussels, London, and Tokyo. The connectors exist between each routing group and are assigned costs based on the network speed and available bandwidth.

Routing topology and costs

If all connections between the routing groups are available, a server in the Seattle routing group always sends a message to the Brussels routing group by sending the message first through the London routing group. This route has a cost of 20, the lowest cost route available. But, if the bridgehead server in London is unavailable, messages originating in Seattle and destined for Brussels travel over the higher cost route, the one that goes through the Tokyo routing group.