AToM – Any Transport Over MPLS

AToM is something, if you know what it can do , you can create solutions that can save thousand of dollars that you might invest in additional links and network infrastructure.  From a engineer perspective , it is something you will love to know and admire . Its an application on MPLS and provides and evidence how MPLS has revolutionize the network world and provides solutions which will be used more and more in coming years . Time has gone when Service Provider/Telecoms provides pure layer 2 dedicated LL and customer has to pay a lots for a international Leased Circuit and he never uses it up to its full capacity . So its time to save your precious dollars by sharing a common infrastructure and enjoy same service Level Agreement.

What is it ?

–  Any Transport over MPLS (AToM) is a solution for transporting Layer 2 packets/frames over an Layer 3 MPLS backbone.

–  Think of it as a method of emulating a layer 2 circuit over an MPLS backbone similar to AAL1 on an ATM Backbone.

–  AToM Supports the following Services

  • Frame Relay
  • ATM AAL5
  • ATM Port Mode
  • Ethernet VLAN
  • PPP
  • HDLC
  • Sonet/SDH

You can imagine a scenario when you have a Layer 3 backbone and you need to provide L2 circuit to your client using that L3 infrastructure . The challenge is how you will transport Ethernet frames received on one leg of an router to another router leg on another side and you have multiple routers in between as well .  Sounds interesting ?

Why Use It ?


–          Savings in transmission costs by consolidating multiple lower speed circuits into a few high speed circuits.

–          Flexibility with available capacity, by having all physical capacity on a single IP/MPLS backbone we can utilize available capacity for the services that require it.


–          SPOF

–          More Overhead

–          Synchronization could be an issue.

How does it work ?

–      AToM uses a two-level (Inner for Service and outer for Transport) label stack similar to a L3VPN.

–      PE’s use targeted LDP sessions to exchange label information.

–      Traffic is received at the ingress PE (AToM start point) and the layer 2 headers are removed.

–      An MPLS label is added suggesting the remote end of the pseudo wire.

–      A second label may be added for the outbound interface.

–      For port mode ATM Without cell packing, the 53 bytes ATM Cell (minus the HEC) is encapsulated in a 64 bytes AToM MPLS Packet.

–      Cell packing is the feature used to conserve bandwidth on the backbone by sending multiple ATM cells in a single IP packet.

One of challenges that arises and  here is added overheads due to this encapsulation . All service providers who have implemented  have faced this challenge to make sure that their core backbone is supporting the MPLS packets with increased MTU .

Let see how much over head is added

Pitfall : Avoid exceeding the Core MTU

Transport Type Header Size
ATM ALL5 0-32 bytes
Ethernet VLAN 18 bytes
Ethernet Port 14 bytes
Frame Relay Dlci (Cisco Encapsulation) 2 bytes
Frame Relay Dlci (IETF Encapsulation) 8 bytes
HDLC 4 bytes
PPP 4 bytes
  • The AToM Header is 4 bytes but it’s required for ATM AAL5 and Frame-Relay. (optional for Ethernet, PPP and HDLC)
  • Label number is 2 if P routers are directly connected. 3 if not.
  • If FRR is requested, it will add another level of tag.
  • Rule: Always assume we need 4 labels.
  • The Label size if 4 bytes

ie. For FR IETF  MTU = 4470 – 8 – 4 – (4 x 4) = 4442 bytes

What is needed ?

–          An Operational MPLS network

–          Targeted LDP session between PE end point routers. (used for advertising Vc labels)

–          TE Tunnels between PE end points.. Question do we need the Tunnels ? If so why ?

–          ESR, exception to use TE Tunnel.

–          Pseudowire configuration.

–          MTU considerations



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The ingress PE router PE1 receives the packets and  attaches the VC label (label 33) onto the frame first. Then it pushes the tunnel/transport label which is 121. The tunnel/transport label is the one  that is  Interior Gateway Protocol (IGP) prefix of remote PE . This prefix is specified by the configuration of AToM. The MPLS packet is then forwarded to connected P router and then it is forwarded by same method , hop by hop, until the packet reaches the egress PE, PE2.

Notice that when the packet reaches the egress PE, the tunnel label has already been removed by PHP. This is because of the penultimate hop popping (PHP) behavior between the last P router and the egress PE. The egress PE then looks up the VC label in the label forwarding information base (LFIB), strips off the VC label, and forwards the frame onto the correct AC.

The P routers never need to look at the VC label; therefore, they do not need the intelligence to be able to do anything with the VC label, the best part is the P routers are completely unaware of the AToM solution.

Because the tunnel label is simply the LDP or RSVP-learned label, no special label distribution protocol has to be set up for AToM on the P routers. The MPLS backbone normally is already using either label distribution protocol. The VC label, however, needs to be associated with a certain AC and advertised to the remote PE. A targeted LDP session performs this job.

Its fun and interesting if you know how it works, the benefits of this technology are immense!


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