Lc-ATM question

Unanswered Question
Sep 7th, 2008
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Hi Guys,

i am just trying to get mt head around this technology. i am trying to configure a LC-ATM interface on a 3640 router. which is connected to a LS1010 switch. MPLS is running in the network.

here is the configuration.

interface ATM3/0.1 tag-switching

ip address

mpls label protocol tdp

tag-switching atm control-vc 1 64

tag-switching ip

.. i just want to know when do we use MPLS atm vpi 101-103 command

Please note:- 101 and 103 and random numbers . for an example.

and can we use mpls atm control-vci and mpls atm vpi x-y command at the same time . or only 1 command at a time.



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Giuseppe Larosa Mon, 09/08/2008 - 09:28
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Hello Amit,

you actually need to use both:

mpls atm control-vci provides the pvc for signalling: here mpls labels are assigned on demand when traffic needs to be set

mpls atm vpi x-y: provides the VP space where the SVC = MPLS labels are assigned on demand

the first is required for signaling

the second provides the MPLS forwarding resources.

Here you can see the mpls distinction of control plane and forwarding plane that comes from ATM itself.

Hope to help


amitbatra Mon, 09/08/2008 - 14:14
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Hi ,

whats the difference between Mpls atm vpi 101-103

and MPLS atm vpi 101 vci range 33-65000

commands ?

Giuseppe Larosa Mon, 09/08/2008 - 22:35
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Hello Amit,

each ATM cell 53 bytes size has a 5 byte header that contains the fields



a pvc or svc is identified by vpi/vci that can change at each ATM switch hop (just as MPLS labels for LSPs)

So the difference is that with the first command the SVCs built by MPLS signalling plane can have

vpi = 101 or 102 or 103

vci= any possible value

in the second case:

vpi= 101

vci = 33 to 65000

both commands provides forwarding resources but the first one should provide a bigger label (vpi/vci) space

Hope to help



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