
Configuring Dial Services
2-14 114062 Rev. B
Circuit Backup versus Link Backup
When you select PPP or Frame Relay for the backup circuit, you determine
whether the router performs circuit backup or link backup.
Circuit backup is a point-to-point backup connection. One circuit is backed up at a
time, instead of an entire interface. When you back up a point-to-point primary
circuit (Standard, PPP, or a single Frame Relay PVC), the protocol for the backup
circuit is PPP.
Figures 2-
6 and 2-7 show examples of circuit backup. Figure 2-6 shows a
Standard primary circuit aided by a PPP backup circuit. Figure
2-7 shows a PPP
primary circuit aided by a PPP backup circuit.
Figure 2-6. PPP Backup over a PSTN
Note: For circuit backup of a single Frame Relay PVC, the backup line passes
through the switched network only, bypassing the Frame Relay network.
Router
Router
Router
Remote site C
Modem
Remote site B
Central site A
Modem
If either primary line fails, the modem at Central site A
can connect to router B or C via a backup circuit.
PSTN
Modem
DS0020A
Primary line
Backup line
Key
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