Subject: RE: Question related to shim header...

From: manikantan <[email-masked]>
Date: Fri, 5 Nov 1999 18:37:52 +0530

Hi

Shim header or "label stack" (with one label minimum or more than one) is a must in 
all types of L2 media.

In case of Ethernet the shim header will contain labels which are of the Generic label type.
The top most 4 byte in the label stack in this case will have the generic label (20 bits), 
and bits set to indicate the bottom of stack and the associated TTL.

In case of ATM and FR the top most label of the label stack will be either the 
ATM Label (VPI/VCI) or the FR Label (DLCI). In this case too the "data" must
have shim header. However the first 20 bits in the shim header will be null, 
( as the label is indicated already by the VPI/VCI or the DCI), the S bit for
stack depth needs to be indicated as well as the TTL.

For the remaining labels in the label stack ( in case the stack is of depth more
than one) the remaining labels will be of generic label type.

Hope this answers your question

best regards
mani
 
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-----Original Message-----
From:	G.B.Naidu [SMTP:[email-masked]]
Sent:	Friday, November 05, 1999 6:55 PM
To:	[email-masked]
Subject:	Question related to shim header...

Hi

I have a question regarding shim header.

When L2 media is ATM & Frame relay, is shim header must or optional?

If L2 media is ethernet, what will be the case? Shim header is a must or
not?

If it is optional, how we will find out whether shim header is present or
not?

that's all

thanks
-- gbnaidu.

---

From: "G.B.Naidu" <[email-masked]>
Date: Sat, 6 Nov 1999 12:42:20 +0530 (IST)

Hi Mani,

Thanks a lot for the reply. It almost answers my query. The other part of
my query was that how do you identify an MPLS packet. The answer is by
looking at the Ehtertype in case Ethernet is the L2 media. This is
mentioned in the MPLS-ENCAPS draft, page: 19, sec 5.

Any comments

regards
-- gbnaidu.