Subject: Carrying ATM labels in the Label TLV

From: "PHILIP MATTHEWS" <[email-masked]>
Date: Wed, 24 Nov 1999 10:23:27 -0500

[I feel a bit foolish asking this question, because I thought I understood
all the basics about MPLS, but there has been a bit of a debate raging
internally on this issue at my company, so I thought it was worthwhile
clarifying things by asking the mailing list.]

In ATM, the VCI field is 16 bits and the VPI field is 8 bits on a UNI link
and 12 bits on an NNI link. Assuming an NNI link and assuming the VPI/VCI
fields are concatenated to form one label, this gives a possible 28 bits.

However, the Label TLV in LDP and CR-LDP, and the label stack itself allocate
only 20 bits for the label field.

This seems to me to mean that there is a range of VPI/VCI values that
cannot be used as labels because they cannot be specified in the signalling protocol
or used in the label encap.

Is the sentence above true, or am I missing something?

- Philip Matthews
Newbridge Networks Corporation

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From: Eric Rosen <[email-masked]>
Date: Wed, 24 Nov 1999 10:43:35 -0500

Philip> This seems  to me to  mean that there  is a range of  VPI/VCI values
Philip> that cannot  be used as labels  because they cannot  be specified in
Philip> the signalling protocol

Did you miss section 3.4.2.2 in the LDP spec? 

Philip>  or used in the label encap. 

VPI/VCI values  never appear in  the encapsulation specified  in draft-ietf-
mpls-label-encaps, they only appear in the VPI/VCI field of the cell header.

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From: "PHILIP MATTHEWS" <[email-masked]>
Date: Wed, 24 Nov 1999 14:21:46 -0500

Ok. So now I feel foolish....
Thanks for the pointer, Eric!

- Philip Matthews
Newbridge Networks Corp.