The MPLS WG Archive

[James_Huang@Mitel.COM]

From:  James Huang@MITEL on 01/24/2001 12:03 PM PST
Hi Bora:
     Maybe I did not express my concern clear enough.  As you said, the LSP
setup will fail when the egress LSR assigns an Implicit NULL label to the
penultimate hop, if the penultinate hop does not support PHP.   So there should
be some negotiation mechanism in LDP or RSVP/TE such that an egress router will
know not to assign an Implicit NULL label to a penultimate hop that does not
support PHP.   But I can not find such mechanism in the documents.

-- James Huang





Bora Akyol <akyol@pluris.com> on 01/23/2001 08:24:45 PM

To:   James Huang/USA/Mitel@Mitel
cc:   mpls@UU.NET

Subject:  PHP negotiation




You should also read the label encapsulation RFC. What happens
is that the egress LSR assigns an (...) NULL label (I think it
should be Implicit Null) to the penultimate hop, if penultimate
hop does not support this, then LSP setup should fail.

There was a LOOONG discussion on this sometime last year.

Some people think this stuff is really clear. I think that it is
in about three different RFCs, and someone needs to write a BCP
that explains common usage and tricks that network operators
that use MPLS should use.

BTW, this question comes up about once a month on the list,
maybe we should have an MPLS FAQ.

Bora


>>>>> "James" == James Huang <James_Huang@Mitel.COM> writes:

    James> Hi all, The following paragraph is from Section
    James> 3.16. (Penultimate Hop Popping) of RFC3031.


    James>    "Initial label distribution protocol negotiations
    James> MUST allow each LSR to determine whether its
    James> neighboring LSRS are capable of popping the label
    James> stack.  A LSR MUST NOT request a label distribution
    James> peer to pop the label stack unless it is capable of
    James> doing so."

    James>      However, I can not find any mechanism in
    James> LDP/CR-LDP or RSVP-TE to accomplish this PHP
    James> negotiation.

    James>      Am I missing something here?



    James> -- James Huang