
Dial Services Overview
114062 Rev. A 1-15
The Configuration of the Backup Circuit
The configuration of the backup circuit depends on the protocol of that circuit. If
Standard or PPP is running over the primary circuit, the backup circuit inherits the
primary’s network-layer address configuration. Unlike the network-layer address
configuration, the data link-layer configuration can be unique.
If Frame Relay is running over the primary circuit, the backup circuit can inherit
the primary’s configuration or it can use its own configuration. When the Frame
Relay primary and backup use the same configuration, it is called the shared
configuration. The shared configuration allows a backup circuit to use the same
network-layer address as the primary circuit. The data-link layer configuration,
that is, the backup’s PVCs, data link control identifiers (DLCIs), and filters can be
the same or different from the primary circuit.
If the backup uses a unique configuration, this is called the secondary
configuration. A secondary configuration uses a different network-layer address,
PVCs, and filters from the primary for the same Frame Relay interface.
Activating the Backup Line
When the router detects a failure on the primary line, it selects a line from a
backup pool. The router activates a backup line under any one of the following
conditions:
• The primary circuit is not operating.
• All of the primary circuit’s lines have failed.
• An active backup line has failed.
Note: You do not configure different PVCs if the backup circuit connects to a
different interface at the destination. In this case, the switch informs the
destination router about available PVCs.
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