RE: AToM header

You essentially have two layer 2 headers, an outer layer 2 header and an inner layer 2 header. The inner layer 2 header is the one that you are tunneling. It could be Ethernet, Frame Relay, ATM, PPP, etc. This is why it's considered "Any Transport" over MPLS. So suppose your network looks like:

CE1 -- PE1 -- MPLS CORE -- PE2 -- CE2

        Let's assume that the links between the CEs and the PEs are T1s running PPP, and the MPLS core is running Ethernet (10/40/100G, etc.) CE1 pings CE2, and when the frame leaves CE1 and is encapsulated into MPLS by PE1 the packet would look like:

Ethernet | MPLS | PPP | IP | ICMP | ECHO

        The outer Ethernet header is just normal Ethernet. The MPLS header is where your transport and VPN pseudowire labels are, along with the control word. The PPP and the rest of the payload is what's being tunneled. From CE1 and CE2's perspective, they look like they're on the same layer 2 segment, but it's actually a virtual circuit (hence "pseudo" wire) that runs over the MPLS network.

Make sense?

Brian McGahan, CCIE #8593 (R&S/SP/Security), CCDE #2013::13
bmcgahan_at_INE.com

Internetwork Expert, Inc.
http://www.INE.com

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of vishal bhugra
Sent: Friday, April 26, 2013 4:37 AM
To: CCIE GroupStudy; Online Study List
Subject: AToM header

Hello experts

While studying the MPLS I came across the AToM. It states that AToM is basically to provide layer 2 connectivity to customer. When reading the data plane communication of the AToM the explanation given is while packet comes to ingress PE, it will put 2 labels on it VC label and tunnel label.

*Now my Query are as :-

1) MPLS is 2.5 shim header. How can I say label over L2 Frame ?

*
*2) Control Word is a 32 bit field inserted between layer and VC label, again how this is possible based on my above query ?*

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Thanks&regards
Vishal
+91 8587900305
Best way to predict the future ..... is to create it
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Thanks&regards
Vishal
+91 8587900305
Best way to predict the future ..... is to create it
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