This article describes link state routing and how it works in Exchange 2000 Server.
Exchange 2000 determines the route that a message takes based on a least-cost algorithm. Each Exchange 2000 Server computer has a map of the entire messaging topology of which it is a member. This map, which is represented in the link state table, is updated regularly and is propagated to all the servers in the topology, so that each server can determine not only the most inexpensive route to deliver a message, but also whether all the connectors that comprise the route are functioning.
The link state table is used on each Exchange 2000 Server computer to store link state information that is propagated by a link propagation protocol called the Link State Algorithm (LSA). The link state table is used to evaluate the most suitable route for a message given cost and availability information. The link state table is only present in memory and is rebuilt from scratch every time the server is restarted.
The LSA propagates the routing status of the messaging system in close to real time to all Exchange 2000 Server computers in the system. This has the following advantages:
- Each Exchange 2000 Server computer can determine the best routing option at the source and therefore avoid sending a message on a path on which a downstream link is disabled.
- Messages do not bounce between servers because each Exchange 2000 Server computer can determine whether alternate or redundant links are up or down.
- Message looping problems are eliminated.
How the Link State Recovers
After a link is marked as down, the original routing continues to retry the connection at 60-second intervals. Even though no message is waiting to transfer, the routing continues to try to contact the destination server. After a connection is re-established, the routing notifies the local routing group master that the connection is available, and the routing group master notifies all servers in the routing group and routing master servers in other routing groups that the connection is available.
Routing Group Masters
Link state information is most effective when multiple routing groups are configured in an organization, particularly if redundant paths are available. Each routing group has a master server that is fed link state information from different sources. The master keeps track of the link state data and propagates that data to the rest of the servers in the routing group. The master is normally the first server that is installed in the routing group, but you can change the master in Exchange System Manager; navigate to the routing group, click Members
, right-click the server, and then click Set as Master
. When a non-master server receives new link state information, the non-master server immediately transfers the link state information to the master, so that other servers can receive the information about the routing change.
Only two states exist for any given link, up or down. Therefore, connection information, such as whether a link is active or in a retry state, is not propagated and is confined to the server that is involved in the message transfer.