Subject: Why no separate MPLS label/Shim Header when we use MPLS over ATM ??

From: Vikram Gupta <[email-masked]>
Date: Sun, 1 Apr 2001 09:14:49 -0700 (PDT)

Hello Everybody,

I have 2 questions about which I am pretty
confused.......
1.Why is it that sometimes the MPLS  label is  embedded in
the header of the data link layer ( e.g in the ATM VCI/VPI and the
frame-relay DLCI ) and sometimes it is in the shim (between the Layer-2 data-
link header and Layer-3 network layer header, as in Point -to-Point PPP/Ethernet).
Why 2 different mechanisms ?

2.Would it be correct to say that when  ATM is the data-link layer ,then we have
No separate MPLS label/Shim Header to encapsulate the IP Packet.The IP packet is directly encapsulated  in ATM cells . ..and the VPI/VCI
Present in the ATM Cell Header do the work of MPLS Labels.

In such a case what would be the significance of MPLS signaling protocols like LDP /RSVP which co-ordinate Label Distribution and create  LSPs ? 
Would they co-ordinate which VPIs/VCIs  would be used for which LSP ? But how could LDP possibly tell ATM
which VPI/VCI to use for which LSP ??

Sorry if the questions sound stupid...since I am new to this field.
Thanks a lot,
Vik

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