The MPLS WG Archive

[George Swallow <swallow@cisco.com>]

Dave -

> The reason I suggested the text is that there is a converse situation to
> what you underlined where you can have more labels in the stack than FECs if
> you insert the PING (for example) in an LDP network, and due to uniform mode
> TTL handling, a traceroute terminates in a TE trunk then the P LSR in the
> trunk has no knowledge of any FECs in the stack. 

Correct.

> The procedure as described
> will break.

Could you explain how?  I'm not seeing a problem.

> On item 4, my reason for saying the FEC stack has limited utility in
> traceroute mode is that when tracing a VPN or PW, the P LSRs have no
> knowledge of the VPN or PW FECs anyway, and the processing rules would halt
> on matching the first FEC in the stack. So as written, the extra FEC stack
> elements would not be examined at LSRs where TTL exhausts. ergo, not really
> useful.

Yes, but you should never get that far if things are working properly.
If you have 

     FEC Type     Label Stack    FEC Stack

     TE                57           ...
     LDP               22         10.10.10.10
     VPN               45         192.27.168.1

Then the rules say look at first label.  

In step 1. Is there a FEC (stack entry) corresponding to
this label etc, if not go to step 4.

Then you find that you are label switching at depth 3.  The originator
of the ping will be notified that you are doing something at a layer
below 

I'll make it clearer that you are looking for a stack entry in step
one.  In step 4 if you can determine the label operation, ostensibly you
also know that the label is valid on the interface and what the FEC is
(although this last piece is not really necessary for the processing)

...George

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George Swallow             Cisco Systems                  (978) 936-1398
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