When a backup interface is
specified on a primary line, the backup interface is placed in
standby mode. When in standby mode, the backup interface is
effectively shut down until enabled. The backup route between the
two company sites is not resolvable and it does not appear in the
routing table. Example
displays the output of the show
interface dialer 1 command when it is run on the backup
interface of the network displayed in Figure .
The primary link in Figure
will be the only route appearing in the routing table. The branch
office router continues to monitor the two following events:
- Carrier detect
- Keepalives
If the branch-office router does not receive
either a keepalive or carrier-detect response, it assumes that the
primary line is down and activates the backup link. If a primary line
fails, the primary interface comes down and
the backup is enabled. The routing table now reflects the backup
route as the only resolvable route between the two company sites.
If a physical link is used as a backup to a
primary connection, it is in standby mode and cannot be used as a
link to another site.
In Figure ,
Branch Office A wants to back up its Frame Relay connection with
ISDN Basic Rate Interface (BRI). Branch Office A also wants to use the same BRI interface
as a DDR link to Branch Office B, however. If Branch Office A
places the physical BRI link in standby mode, it is deactivated
and does not activate until the primary line fails or reaches a
specified threshold. Thus, the BRI link cannot be used to connect
to Branch Office B.
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