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Advanced DDR Operations 161
Figure 6-10
Rotary Groups
The only protocol or configuration attributes configured on the physical interface are the
individual commands that make the BRI interface a part of the rotary group and the encapsulation.
On dialer pool interfaces, you can set a priority to specify the order in which the interfaces are
used. With rotary groups, that granularity is not possible. All protocol attributes are entered at
the logical dialer interface configuration level.
The use of dialer profiles versus rotary groups comes down to one question: "How much control
do I want to have over the link?" With dialer profiles, a map class can be created and applied
on a per destination basis. This allows a great degree of control over the characteristics of a
particular call based on the destination being called. Rotary groups do not make use of the map
class featureset. Therefore, they are limited to the characteristics applied to the dialer interface.
Dial Backup
Dial backup provides redundancy for WAN links. Although the ISDN connection may not
provide the same amount of bandwidth as a primary link, dial backup provides a maintenance
path if the primary link fails. Once the down or overload condition of the primary link is
detected, the dial-on-demand configuration is placed into service.
Figure 6-11 illustrates a network in which dial backup could be utilized.
The primary data pathway across the WAN exists between each router's Serial 0 interface. As
mentioned, dial backup can be implemented in two ways. The first, and most obvious, manner
is to have dial backup function when a primary link fails. This occurs when a "down" condition
is detected on the primary interface, and then the secondary, or backup, link is changed to an
"up" state and a connection is established.
The backup pathway is used in the absence of the primary, obviously. But, how does the backup
link know when it's time to return to the backup state? Simple--you configure the parameters
of the "up" and/or "down" state for the backup link.
Dialer 2
BRI 0
Router A
BRI 1
BRI 2
BRI 3
Router B
PSTN