What do you mean by "devices?"
If you mean a router connected directly to a frame-relay switch or two
switches directly connected using an NNI connection, then yes, the DLCIs
must be the same for each device sharing the link. The only way a device
can know which VC a frame belongs to on any link is to examine the DLCI
contained in the frame header. If a router sends a frame to the switch with
a different DLCI number than the one the switch is configured for, then the
frame may be delivered to the wrong location or it may be dropped.
OTOH, if you mean two routers connected to each other through a "cloud,"
then the answer is no, the DLCIs usually will be different. The DLCI
identifies the PVC only between the device and the switch is directly
attached to. It is local to that switch's INTERFACE. DLCI = Data Link
Connection Identifier. It identifies an individual virtual connection on a
single data link.
The switch is configured to map the DLCI on a particular interface to a
virtual circuit on an outgoing interface or port, which is usually going to
be identified with an ATM VPI/VCI. The ATM virtual circuits are mapped
through the ATM cloud until they get to the switch at the opposite end,
where (finally) the VPI/VCI of that connection gets mapped back to an
outgoing frame-relay DLCI that connects to the router on the other end.
There's no facility in the standard ATM or frame relay protocols for the
switch at either end of the PVC to communicate to the opposite switch about
its local frame-relay DLCIs defined to customer routers.
There have been numerous discussions of this topic over the last couple of
years, which by now belongs on the CCNA discussion list. I hope this
doesn't start another one! Please search the archives for further
information.
Pamela
At 12:48 AM 3/11/01 -0500, you wrote:
>It was my understanding that two devices in a PVC connection had to have the
>same DLCI number. Is this correct?
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