dlci swap

Unanswered Question

hi @all,

i didn?t find a answer to my question while searching the forums, so i post it

now ;-).

i?m studying for ccna and have a problem understanding, how the

frame-relay-switch swaps the dlci. it might be, because there is no detailed

diagram of the lapf header, where the dlci is included, in the oficial self

study guide from ciscopress. is there a combined "src dlci" and "dst dlci" in

the lapf header? referring to the ciscopress bookthere?s only a single dlci

field. so if it?s the dst which is in the dlci field, how does the "swapping

switch" know what source the frame came from?

thx in advance



I have this problem too.
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mohammedmahmoud Tue, 06/19/2007 - 01:31


The FR switch doesn't swap the DLCI automatically, the Service Provider is responsible for the end-to-end PVC, by simply cross-connecting the PVC end-to-end (from the source DLCI to the destination DLCI) by manual configuration on the FR switches.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

Richard Burts Tue, 06/19/2007 - 03:18

The Frame Relay switch knows which source the frame came from because it knows the interface on which it was received and it sees the incoming DLCI in the frame. Knowing the inbound interface and DLCI value is sufficient to accurately identify the source. The Frame Relay switch then uses its configuration logic (managed by the Frame Relay service provider) to determine the outboud interface and DLCI value for the frame. The switch then swaps the inbound DLCI with the outbound DLCI.




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