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Question on Exchange 2003 Routing Groups

Posted on 2010-01-06
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Hi

I have a question on Exchange 2003 Routing Groups that has been bugging me.

Let's say I have three routing groups - NY, LA, and London.

Network wise, there is a WAN link from NY to London, and London to LA, but not one from LA to NY.

My routing group connectors are set up so that:

i) 1 x connector from NY > London : Cost 10
ii) 1 x Connector from London > NY: Cost 10
iii) 1 x Connector from London > LA: Cost 10
iv) 1 x Connector from LA > London: Cost 10
v) 1 x Connector from LA > NY: Cost 50
vi) 1 x Connector from NY > LA: Cost 50

Now, due to least cost routing, mail from NY to LA should travel via London, am I correct?

However, what if the London Exchange bridgehead goes down (NB: the WAN is still up)? The alternative routing path for NY > LA is direct, but what if there is no direct WAN link? Would Exchange know to route the mail via the London network even though it wasn't routing via the London bridgehead?
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Question by:kam_uk
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by:tigermatt
ID: 26189110

>> Now, due to least cost routing, mail from NY to LA should travel via London, am I correct?

Yep, that looks to be correct.

>> However, what if the London Exchange bridgehead goes down (NB: the WAN is still up)? The alternative routing path for NY > LA is direct, but what if there is no direct WAN link? Would Exchange know to route the mail via the London network even though it wasn't routing via the London bridgehead?

Exchange doesn't have a clue what your underlying network infrastructure is. This is in the same way that Active Directory doesn't know what the underlying WAN link is when you start looking at Active Directory Sites and Services. The fact you normally configure Exchange and Active Directory to match your underlying network layout is pretty much irrelevant.

So, in this case, if Exchange can't find a route back to London and needs to fail over to communicating directly from LA to NY, it'll simply route itself via the WAN link to London. Your routers know where traffic for New York's subnet has to go and will simply route it to NY accordingly.

-Matt
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by:kam_uk
ID: 26189137
Thanks Matt.

In which case, would it be better to change the routing so that mail from NY to LA goes 'direct' (from Exchange's point of view) anyway - regardless of whether the London bridgehead was up or down? Even though mail actually routes via the London routers, there is no need to travel via the London bridgehead?
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tigermatt earned 2000 total points
ID: 26189324

>> Even though mail actually routes via the London routers, there is no need to travel via the London bridgehead?

That ultimately depends on why you were routing via London. If you were doing anything special to the mail on your London servers, then clearly you'd still want that to happen. However, if it was routed via London simply to match your underlying network configuration, there's no special reason that cannot be changed so NY talks directly to LA. Your WAN links to London will still have the traffic passing over them, but it would reduce some load on the London Exchange Servers.

-Matt
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