Site B, however, can pass SMTP traffic to either
site. In this case, Site B should be designated as a Hub site to allow message traffic to be routed successfully.
??‘
Exchange Server 2007 Routing Group
(DWBGZMFD01QNBJR)
Routing Group A Routing Group B Routing Group C
RGC
Cost = 10
RGC
Cost = 10
RGC
Cost = 10
RGC
Cost = 10
RGC
Cost = 10
Figure 11 - 2
Chapter 11: Exchange Server 2007 Routing
305
Hub sites must exist along the least cost route between the two Hub servers in order for mail to be
routed there. In other words, just designating a site as a Hub site does not guarantee all mail will route
through it. It must already be a possible hop along the calculated path. Creating Hub sites is discussed
later in this chapter in the ??? Working with Active Directory Sites ??? section.
Backoff
If direct relay fails to all the Hub servers in the destination site, Exchange performs a backoff routine. The
backoff routine tries to determine a new delivery point as close to the point of failure as possible. Remember,
link state information is not stored, so it retries connection attempts to the unavailable site. The message
queue is automatically put into a retry state and attempts redelivery based on message retry interval
settings. If there are more than four hops to the destination, a slightly different process called binary backoff
is used.
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