Using Demand Routing For Backup Connections - HP 7102dl - ProCurve Secure Router Configuration Manual

Configuring Backup WAN Connections
Determining a Backup Method
3-12
You can configure a persistent backup connection, which is initiated
immediately if a backup condition occurs on the primary connection and
stays up until the primary connection is available again.
Before you configure a backup connection, you should evaluate your network
environment and then determine which option best meets your company's
particular needs.

Using Demand Routing for Backup Connections

Demand routing allows you to capitalize on the main advantage of a dial-up
connection: it establishes the dial-up connection when it is needed and
terminates the connection when it is no longer necessary. For example, you
may lease an ISDN line to serve as the backup WAN connection between the
main office and a branch office. If the primary interface goes down and no one
is transmitting traffic, you may not want the backup WAN connection to
become active. This type of usage would substantially increase your com-
pany's telephone costs. Instead, you may want to establish the ISDN connec-
tion only when two conditions are met:
the primary interface goes down
traffic must be transmitted between the two offices
Demand routing only establishes the backup connection when traffic is sent
from the main office to the branch office and the primary interface is unavail-
able. (See Figure 3-3.)
In addition to establishing the connection only when it is needed for data
transmission, demand routing ensures that when the dial-up connection is idle
for certain amount of time, the ProCurve Secure Router terminates the call.
You can configure the idle timer to match the rates you are charged for the
ISDN line. For example, if your service provider charges your company for
every two minutes that the ISDN line is established, you can set the idle timer
to 110 seconds. The ProCurve Secure Router will then disconnect the ISDN
line when it has been idle for 110 seconds, and your company will not be
charged for an additional two minutes.
With demand routing, you can also be very selective in the type of traffic that
causes the router to initiate the ISDN connection. For example, you can limit
this "interesting" traffic to packets sent from one subnet to another subnet.
You can also exclude routing updates (if you are using a routing protocol) and
other traffic that you do not think is essential. Carefully selecting the type of
traffic that triggers an ISDN connection limits the amount of time that your
company uses its ISDN connection, thereby decreasing costs.