The MPLS WG Archive

["Cheenu Srinivasan" <cheenu@alphion.com>]

Bruce Nepple rote:
> http://www.ietf.org/internet-drafts/draft-ietf-mpls-lsr-mib-07.txt
>  describes mplsXCLabelStackIndex as identifying a stack of labels
> to be pushed beneath the top label.  The "pushed beneath" concept
> confuses me.  Does this assume the top label is later stripped, or
> am I missing something completely.

Here are the 3 scenarios:

1. incoming label L1 is replaced with an outgoing label L2:
     mplsOutSegmentPushTopLabel = true
     mplsOutSegmentTopLabel     = L2
     mplsXCLabelStackIndex      = 0

2. incoming label L1 is popped and packet is forwarded with
   whatever label stack is underneath:
     mplsOutSegmentPushTopLabel = false
     mplsOutSegmentTopLabel     = don't care value
     mplsXCLabelStackIndex      = 0

3. incoming label L1 is replaced with a stack of more than
   one label: {L2, L3, L4, ... }; L2 is new top label:
     mplsOutSegmentPushTopLabel = true
     mplsOutSegmentTopLabel     = 
     mplsXCLabelStackIndex      = x (entry into label stack table)
   --
   Corresponding label stack entries:
     entry (x, 1): mplsLabelStackLabel = L3
     entry (x, 2): mplsLabelStackLabel = L4
     ...

Cheenu