Tuesday, May 22, 2007

Routing Topology in Exchange 2007 & no more Link State

Exchange 2007 is using Active Directory site Topology to determine how messages are transported in the organization. Exchange 2007 takes advantage of the existing Active Directory site topology to eliminate separate Exchange routing topology. The Active Directory IP site links and the costs associated with them are used to calculate the least cost route between Hub Transport servers in different Active Directory sites.

Each Active Directory site that contains one or more Exchange 2007 Mailbox servers must also have at least one Hub Transport server. The Hub Transport server uses the Active Directory Topology service to retrieve the Exchange organization's configuration information and computes an implicit intra-organizational Send connector that is used when transporting messages from site to site. This topology is only updated when configuration changes occur. The result is minimized traffic related to Exchange. 

By default, the Hub Transport server always tries a direct connection to a Hub Transport server in another Active Directory site. Messages in transport do not relay through each Hub Transport server in a site link path. However, Hub Transport servers in intermediate Active Directory sites along the routing path may perform message relay in the following scenarios:

  • Direct relay between Hub Transport servers will not occur when a hub site exists along the least cost routing path. You can configure an Active Directory site as a hub site so that messages are routed to the hub site to be processed before the messages are relayed to the target server. Hub sites are discussed later in this topic.
  • Exchange 2007 uses the routing path derived from IP site link information when communication to the destination Active Directory site fails. If no Hub Transport server in the destination Active Directory site responds, message delivery backs off along the least cost routing path until a connection is made to a Hub Transport server in an Active Directory site along the routing path. The messages are queued in that Active Directory site and the queue will be in a retry state. This behavior is called queue at point of failure.
  • The Hub Transport server can also use the IP site link information to optimize routing of messages that are sent to multiple recipients. The Hub Transport server delays bifurcation of messages until it reaches a fork in the routing paths to the recipients. The bifurcated message is relayed to each recipient destination by a Hub Transport server in the Active Directory site that represents the fork in the individual routing paths. This functionality is called delayed fan-out.


  • A Hub Transport server must be able to communicate directly with a global catalog server to perform Active Directory lookups.
  • Mailbox servers should be located in the same site as a Hub Transport server. We recommend that you deploy more than one Hub Transport server in each Active Directory site to provide load balancing and fault tolerance.
  • Unified Messaging servers submit messages to a Hub Transport server for transport to a Mailbox server. A Unified Messaging server may be located in a hub site or near the IP/voice over Internet Protocol (VoIP) gateway or IP Private Branch eXchange (IP/PBX). The Hub Transport server that has the same site membership as the Unified Messaging server will receive messages for transport and route the messages to other Hub Transport servers and Mailbox servers in the organization.
  • Client Access servers provide a connectivity point to the Exchange organization for users who are accessing Exchange remotely. A Client Access server must be deployed in each site that contains Mailbox servers. The Client Access server lets the user connect directly to the Mailbox server to retrieve messages, but any messages that are sent from the remote client must be transported through the Hub Transport server


Unlike earlier versions of Exchange, Exchange 2007 does not use a link state routing table and does not try to calculate an alternative route when a connection is unavailable. This eliminates link state communication between Exchange servers and creates a more deterministic routing topology

Direct relay   Exchange 2007 relies on the underlying network infrastructure to transport a message. In the Exchange 2007 organization, messages are relayed directly from the source server to the target server, reducing the number of hops a message takes during delivery. When routing resolution occurs, the name and IP address of the destination server is resolved. If multiple IP site links exist between the source and destination, the route calculation is used to determine the optimal point for message bifurcation and the point at which to queue should delivery be unsuccessful. With direct relay, intermediate Hub transport servers don't process messages.

Hub sites   For administrators who require more control over Exchange routing, we have provided features that enable you to modify the default direct relay behavior. You can specify that an Active Directory site is a hub site. A hub site is an Active Directory site through which all messages to be relayed through the Hub Transport servers are forced to pass. The hub site must exist along the least cost routing path between the source and target servers. This configuration is especially useful for network environments that have firewalls between sites that may prevent successful direct relay.

Site link cost override   For even more control over message routing behavior, you can assign an Exchange-specific cost to Active Directory IP site links. By default, Exchange calculates the least cost route between Active Directory sites by using the costs assigned to those links for the purposes of determining Active Directory replication topology. If these costs don't provide the optimal Exchange routing behavior, you can use cmdlets in the Exchange Management Shell to set an Exchange-specific value to the IP site link. 

Queue at point of failure   In earlier versions of Exchange, when a target server was unreachable, the down connector state was propagated throughout the Exchange organization by link state updates, and an alternative route was calculated. In Exchange 2007, when a message can't be relayed directly to the target server because of network problems, no alternative route is calculated. The message queues on a Hub Transport server in the closest reachable site to the point of failure. Using the least cost routing path calculated at startup, message delivery backs up along the path of intermediate sites until delivery to a Hub Transport server is achieved. When the network problem is resolved, or configuration changes update the routing table, message delivery resumes to the target site. This behavior helps administrators to better determine the source of network problems.

Delayed fan-out   A message sent to more than one recipient must bifurcate, or split, to be delivered to more than one destination. Exchange 2007 delays this bifurcation until it reaches a fork in the routing paths. By delaying bifurcation of the message, bandwidth consumption is reduced.

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