Release Notes for Router Software Version 11.00 Revision 4n
114968-A Rev. A 27
Using Demand Circuits for Backup
You can now configure a demand circuit to act as a back up for a failed
point-to-point circuit. If the point-to-point circuit fails, the router immediately
establishes a demand circuit back to the original router and data transmission can
continue. The demand circuit can operate independently of the point-to-point
circuit, so the demand circuit can remain active after the point-to-point recovers.
This type of configuration is recommended only if you want to keep the demand
circuit active after the point-to-point circuit recovers.
The following diagram shows how this configuration works.
In the diagram, Router A is connected to Router B via a point-to-point link. This
link, connected at port S21, can support any WAN protocol that the router
supports. Router B has a demand circuit configured at S23. It also has the backup
circuit parameter set to the circuit number of the point-to-point link S21. If the
point-to-point link fails, Router B activates the demand circuit.
Making the Configuration Work by Configuring an Unused Port
Typically dial backup service provides backup for a failed primary connection. In
this case, you cannot use dial backup service because the circuit will not stay
active once the point-to-point connection recovers. Instead, you configure another
port on the same router as the point-to-point circuit for dial backup service. You
make this port an unused primary interface set for slave mode, so it does not
initiate calls.
The backup does not activate until the remote router makes a demand connection.
When a call comes in to this disabled port, the backup circuit comes up on the
backup interface.
Router A
Router B
S21
S22
S21
S23 - Demand Cct.
1.1.1.0 - Circuit 1
2.1.1.0 - Circuit 2
.1
.1
.2
.2
Router A - Dial Backup Service
Router B - Demand Service
Backup for S22 - S23
