Datasheet

Designing Organization Configuration to Meet Routing Requirements

You might have hub sites if a firewall prevents direct communication between certain AD sites

or if a company policy exists where all message traffic must be routed through a special AD site.

A hub site is considered only when it lies on the least-cost routing path calcu-

lated by the Hub Transport server. Before you implement hub sites, it is impor-

tant that you review your Active Directory topology to make sure that the least-

cost routing path always includes the AD sites you want to define as hub sites.

You can configure hub sites using the Exchange Management Shell and the Set-AdSite

command. You have to do this site by site, so keep track of what changes you made!

The following command shows an example where I set the hub site to the AD site Site2. You

have to be Exchange Organization Administrator to configure this.

Set-AdSite -Identity “Site2” -HubSiteEnabled $true

Modifying IP Site Link Costs

The Active Directory site topology might not be optimum for Exchange message routing in spe-

cific cases. For that reason there is a way to modify the least-cost routing path by modifying the

cost of IP site links. Doing so will add an Exchange-specific cost to the IP site link but will not

modify the current setting in Active Directory costs. Of course, if you set an Exchange cost, this

overrides the Active Directory cost for message-routing purposes.

After considering your Active Directory site topology and placing your servers

in the right sites, you should carefully consider if you need to implement

Exchange-related IP site link costs, as they are quite hard to manage.

The following Exchange command-shell command assigns an Exchange-specific cost of

20 to the IP site link called Link3-4:

Set-ADSiteLink -Identity “Link3-4” -ExchangeCost 20

Delayed Fan-Out

In a message that is addressed to multiple recipients, the routing technology I explained before

would mean that a copy is created for every recipient. However, Exchange Server 2007 uses a tech-

nique called delayed fan-out to preserve bandwidth when routing messages with many recipients.

After each recipient has been resolved by the Hub Transport server, Exchange Server 2007

compares the routing path for each recipient. The splitting of messages into multiple copies does

not occur until a Hub Transport server is reached, which splits up the routing path. Microsoft

calls such a Hub Transport server a fork in the routing path.

For example, we have a message addressed to one recipient in every AD site, Site 1, Site 2,

Site 3, and Site 4. From an Active Directory standpoint, they are all connected sequentially,

Site 1 to Site 4. The first message will get transferred to Site 2 as a single message where the Hub

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