
Configuring Dial Services
1-10 114062 Rev. A
Circuit Backup Versus Link Backup
When you select PPP or Frame Relay for the backup circuit, you are determining
whether the router performs circuit backup or link backup.
Circuit backup describes 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 over the backup circuit is PPP.
Figures
1-5 and 1-6 show examples of circuit backup. Figure 1-5 shows a
Standard primary aided by a PPP backup circuit.
Figure 1-6 shows a PPP primary
circuit aided by a PPP backup circuit.
Figure 1-5. PPP Backup Across a Public Switched Telephone Network (PSTN)
Note: For circuit backup of a single Frame Relay PVC, the backup line passes
only through the switched network, going around 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.
Primary Line
Backup Line
PSTN
Modem
DS0020A
Key
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