The MPLS WG Archive

["Anil Gunturu" <anilg@acc.com>]

Is there any RFC that defines the VPI/VCI for an explicit null label incase of ATM?

Thanks,

-Anil

-----Original Message-----
From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of Eric Gray
Sent: Tuesday, April 17, 2001 6:01 AM
To: Eyad Saheb
Cc: MPLS Mailing List
Subject: Re: Explicit Null Label

Eyad,

    Forgiven. :-)

    There is no need to identify the L3 protocol if the penultimate hop
has already done so.  Also, with the pointed exception of explicit
NULL labels, no label may be used that was not explicitly issued
by the LSR that receives them.  Therefore, only a very lame LSR
would issue a label which it will have to pop without knowing what
L3 will be underneath it.  In that sense, every LSR may create as
many 'explicit NULL' labels for as many underlying encapsulation
protocol/formats as it is capable of supporting.

    The inability of the penultimate hop to 'pop the stack' is exactly
one of the other examples implied in my earlier reply.  I believe in
trying the short answer first.

--
Eric Gray

You wrote:

 

Hi,

        Forgive my ignorance, but Explicit Null can still be used for its initial purpose: identifying the L3 protocol type.

        Also, and I'm not too versed in ATM, but it might be useful for switches which cannot pop the stack.  Thus the edges would send an Explicit Null mapping  if they are the egress for an LSP.

Eyad

-----Original Message-----
From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of Eric Gray
Sent: April 17, 2001 8:16 a
To: anilg@acc.am.ericsson.se
Cc: mpls@UU.NET
Subject: Re: Explicit Null Label

Anil,

    It is not necessary to preserve the value of the EXP bits
using the explicit NULL label if the penultimate hop is able
to map the bits to a corresponding DSCP in the IP header.
But this is just one example of what the penultimate hop
might not be able to do, thus creating a need for explicit
NULL labels...

--
Eric Gray

You wrote:

> Is there any other purpose for an explicit null label other than preserving
> the Exp field for QoS purposes?
> Thanks,
> -Anil
>
> Anil Gunturu, Software Engineer
> Ericsson, Inc                           phone: (805) 562 6431
> Datacom & IP Services                   email: anilg@acc.am.ericsson.se
> 70 Castilian Dr, Santa Barbara, CA-93117