Link State Routing

Within each Microsoft Exchange 2000 Server routing group exists a routing master—a server that you've nominated to collate updates and generate Link State Tables (LSTs). (The first server you assign to a routing group is automatically the routing master—until you select another server to fill the role.) Through bridgehead servers, the routing master also updates its counterparts in other routing groups to maintain a dynamic picture of routing conditions for the entire messaging network. Each routing master uses the Link State Algorithm (LSA) to determine the best way to route messages given the current state of the network and the available connectors. (LSA is a modified version of Dijkstra's algorithm, which is a commonly used method to determine the shortest path between two points in a network. Network routers often use Open Shortest Path First—OSPF—another variant of Dijkstra's algorithm, to decide the most efficient manner in which to route network packets.)

Link-state updates from other servers in the same routing group and from routing masters in other routing groups provide the raw data that the routing master uses. Within a routing group, servers bind to port 691 to send link-state updates to the routing group master. Communication occurs over a special LSA protocol that Microsoft developed specifically for this purpose. To send link-state updates among routing groups, Exchange 2000 uses the X-LINK2STATE Extended SMTP (ESMTP) verb, which Routing Group Connectors (RGCs) and SMTP connectors support. X.400 connectors can also pass link-state information. (For more detailed information about Link State Routing, see "Exchange 2000 and SMTP," March 2000.)

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