Routing loops in Exchange 2003/2007 mixed mode

kam_uk used Ask the Experts™
Hi All

From what I understand, minor link states need to be suppressed on all Exchange 2003 servers if more than one legacy routing group connector is added to the Exchange org.

I just cannot get my head around how these routing loops can occur though! I know E2007 doesn't use link state, but how can that form routing loops, and where would the routing loops exist?

Would anyone be able to suggest an example that would clear this up?
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thats an architectural difference between exchange 2007 and legacy versions.
In simple terms exchange 2003 without "minor link state supression" would not be able to detect a routing group connector down and would continue to send emails over that connector,this results in a loop as the messages are held up.
see this article
IT Infrastructure Architect
Ex2k7 does not use the same technology (link state) which routing group uses except if it is installed in an Ex2k3 coexistence environment. Exchange 2007 always installed in one and only one routing group which is dedicated to exchange 2007. And it automatically creates the connectors during the installation.

In a pure exchange 2007 environment uses active directory sites as its method of routing.

If you are talking about disabling the link state update, you don't really need to suppress link state updates unless you create more than one routing group connector.

Following paragraph taken from below link 

All messages that are relayed between Exchange 2007 and Exchange 2003 are routed through the initial routing group connector. This can create excessive routing hops as more Exchange 2007 servers are deployed in additional Active Directory sites. Exchange 2007 servers in all sites are considered members of the same routing group. For example, suppose you have routing groups in Hong Kong, London, and Chicago. If your first Exchange 2007 server is deployed in Chicago, it makes sense to establish the first routing group connector to a bridgehead server in Chicago. However, if you then deploy an Exchange 2007 server in Hong Kong, when messages are sent from users whose mailboxes are on Exchange 2003 servers in Hong Kong to users whose mailboxes are on Exchange 2007 servers in Hong Kong, the messages will be routed through Chicago.

In this case you need create an additional routing group connector and suppress the link state to stop messaging routing towards Chicago site.

Following paragraph taken from below link 

When only one routing group connector is established between Exchange 2003 and Exchange 2007, you do not have to make any changes to link state, and routing loops will not occur. However, if more than one routing group connector is configured between Exchange 2003 and Exchange 2007, the minor link state updates that are transmitted between Exchange 2003 servers can introduce problems. When Exchange 2003 detects that a connector is unavailable, link state updates are communicated throughout the Exchange organization to notify them of the connector down state. The Exchange 2003 bridgehead server also tries to determine an alternative route for message transfer to the destination server. However, Exchange 2007 does not use link state to determine a routing path. The Exchange 2007 Hub Transport server will be unaware of the down connector state and may decide to route a message back through a routing path that Exchange 2003 is trying to route around.

Exchange 2003 may try a routing path other than the least cost routing path when it detects that a connector is down. However, Exchange 2007 will always use the least cost route, introducing the potential for a routing loop to occur.

To avoid routing loops, you must suppress minor link state updates before introducing additional routing group connectors. Minor link state updates are sent between Exchange 2003 servers to update the link state routing table to indicate that a connector is down. When the SuppressStateChanges registry key is set, you are turning off the ability for a connector to be marked as down. Link state messages are also used to notify Exchange 2003 servers of configuration changes to the Exchange organization, such as the addition or removal of a connector or a server. When you suppress minor link state updates, it does not prevent these major link state update messages from being communicated.

When minor link state updates are suppressed, Exchange 2003 also only uses least cost routing. This eliminates the chance for routing loops to occur. We recommend that you suppress link state updates on every Exchange 2003 server in the organization to maintain a consistent configuration.

Please let me know if you need any further clarification.

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