The MPLS WG Archive

[Alia Atlas <aatlas@avici.com>]

At 09:14 AM 11/20/2002 -0500, Thomas D. Nadeau wrote:
>At 10:40 PM 11/19/2002 -0500, Alia Atlas wrote:
>>At 05:45 PM 11/19/2002 -0500, Thomas D. Nadeau wrote:
>>>         Not necessarily. I think you can test/trace the paths using LSP 
>>> ping,
>>>but of course, there is a price to pay for that too; there is no free lunch.
>>>The advantage of the latter is that it works with all of the hardware
>>>deployed today.
>>
>>How does it work today with the pseudo-wire case?  From the draft, it 
>>seems that the same PW label is used, but it is not the bottom of stack, 
>>b/c there is an explicit NULL underneath.
>>
>>So, suddenly one must forward based not only on the MPLS label but also 
>>the  bottom-of-stack bit...
>>
>>Tell me I'm missing something here, but if the above is really what is 
>>intended in the draft, I don't understand how you can claim that it is 
>>compatible with current MPLS hardware.
>
>         It is my understanding that tracing the PW "tunnel" LSP is the 
> same as any
>other MPLS LSP.
>
>         --Tom

To quote from the current draft:

  "To test an LSP that carries non-IP traffic, before injecting ICMP and
    MPLS ping messages into the LSP, the IPv4 Explicit NULL label should
    be prepended to such messages. The ingress and egress LSR's must
    follow the procedures defined in [LABEL-STACKING]."

Thus, as I understand it, one would have the tunnel label followed by the 
VC label followed by the IPv4 Explicit NULL label.  Unfortunately, at the 
Pseudowire egress, the ingress interface would look at the VC label and 
forward it to the egress interface to be transmitted as a layer 2 
frame.  To avoid this, one would have to look at the bottom-of-stack bit...

So, this sounds to me as if part of the forwarding decision is being made 
based upon the BOS bit.

Alia