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By enabling orphan circuits, you allow the router to forward packets over circuits
you did not configure. Packets that would normally be dropped are now forwarded.
An orphan switched virtual circuit is an SVC that is not configured for your router
but is created when a call-in is received for it. This is similar to Figure 23. However,
Q.933 messages are used instead of LMI to generate the circuit and associate the
appropriate parameters with it. Orphan SVCs are treated the same as configured
SVCs except that you may enable or disable their use with the call-in option of the
enable switched-virtual-circuit command.
Configuring PVC States to Affect the Frame Relay Interface State
You can control the operation of your Frame Relay interface by
1. Enabling the “No-PVC” feature or
2. Configuring “required PVCs” or
3. Configuring “required PVC groups”.
By enabling the Frame Relay “No-PVC” feature, the Frame Relay interface
becomes inactive when there are no active PVCs on the interface. If at least one
PVC is active, the Frame Relay interface becomes active when a successful LMI
exchange occurs between the router and the FR switch.
You can configure a PVC as a “required PVC”. If a PVC is required but not in a
group, the Frame Relay interface becomes inactive when the PVC becomes
inactive. When the PVC becomes active, the interface is activated following a
successful exchange of LMI frames between the router and the Frame Relay
switch.
DLCI 19
Router
B
Frame
Relay
Backbone
Router
D
Router
A
DLCI 18
physical connection
logical connection
Orphan circuit
(Configured in that
direction only)
Figure 23. Orphan Circuit
Using Frame Relay
Chapter 30. Using Frame Relay Interfaces 391
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