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Cell Relay Retreat>MPLS WG Archive>month:2001-Aug> msg00432



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MPLS over FR.

  • From: Shahram Davari <Shahram_Davari@pmc-sierra.com>
  • Date: Fri, 31 Aug 2001 07:07:42 -0700

Hi,

Comments in-line:

> -----Original Message-----
> From: Binita Jha [mailto:BinitaJ@netbrahma.com]
> Sent: Friday, August 31, 2001 9:26 AM
> To: 'mpls@uu.net'
> Subject: MPLS over FR.
> 
> 
> I have a doubt regarding MPLS over FR.
> 1) RFC 3034 says that the top label (DLCI) carries implicitly 
> information
> about the network protocol type. but how does it indicate 
> about if the shim
> header is present or not?

The shim header MUST always be present, even if the packet has one label. Only the S, EXP and TTL values of the top label of the shim header are used. The top label's 20-bit label value is not used at all, rather the DLCI is used instead.

> 
> 2) Also if the shim header is present then how do we know abt 
> what network
> protocol is present?

A DLCI value is bound to a specific network protocol (could be done by partitioning of DLCI space). If the S=1 in the top label, then it means that the payload has the same network protocol. If the S=0, then it means that another label is present and the payload is MPLS.

Yours,
-Shahram

> 
> Kindly send me the answer if anyone has.
> Regards
> Binita
>