Category: AToMPLS

Inter-AS L2VPN MPLS Solutions

  • Option A:
    • each SP treats the other as CE
    • PW terminates as ASBR.
    • The link between ASBR is a AC instead of a PW
    • Granular QoS control between ASBR
  • Option B:
    • PE and P LSR do not learn remote PW endpoint addresses
    • Only the IP for PW endpoint (LER) that are ASBR leaked between ASs
    • T-LDP between ASBR of the SPs
    • PW terminates at ASBR. ASBR in turn build a new PW to its peer ASBR. This new PW connects the original PWs together
  • Option C:
    • PE loopback prefix and labels are exchange between two AS using MP-eBGP on ASBR
    • Instead of terminate the PW on its own ASBR, the PW is terminated on the remote PE on another AS. In another word, the PW is extended end to end between the two ASes
  • Summary:
    • All Inter-AS models (A,B,C) are supported to carry VPWS or VPLS PWs
    • IOS supports LDP for signaling, BGP for¬†Auto-discovery¬†(VPLS)
    • IOS XR supports both LDP and BGP signaling
    • Option B is not supported for BGP signal PWs
    • Per-PW QoS is not support
    • AC interworking is supported in IOS XR
    • L2VPN over TE or GRE is supported

AToM Highlights

What does it do?

  1. To forward layer 2 frames over MPLS core using point to point Pseudo Wire
  2. AToM is Cisco implementation of VPWS (Virtual Private Pseudo Wire)

What are the applications?

  1. For Service Provider, to provide layer 2 services to customers using MPLS core
  2. For Enterprise, to expend layer 2 links or network across data centers

How do it work?

Control Plane:

  1. PE exchange PW MTU as part of PW signaling, the PW MTU is derived from AC MTU, both PE MUST agree on the MTU

Forwarding Plane:

  1. frames are encapsulated in PW TAG and than PE TAG and forward across MPLS core via PW within LSP


  1. Support many L2 protocols: ATM, Ethernet, Frame Relay, PPP, HDLC, TDM (T1, T3)


  1. only support point to point PW
  2. fragmentation is NOT supported. If incoming frames is larger than MTU on the AC, PE will drop the frames
  3. There is NO mechanism to check MPLS core MTU, need to design Backbone MTU to support all AC MTU

Advantages of AToM for SP

  1. Do not need to run legacy networks. Legacy L2 services like Frame Relay and ATM can be transported over MPLS.
  2. Provision L2 services to customers without involving in their L3 IP network.
  3. No changes are needed on P routers to run AT0M on a MPLS network. Only changes are on PE because all AT0M intelligence is all the PE. In face the P LSR do not even know they are switching L2 protocols.


  1. Ethernet over MPLS is a point to point technology. Use VPLS for multiple access Ethernet over MPLS
  2. EoMPLS has two modes:
    1. Ethernet Port mode
      1. Transports whole ethernet port, the ethernet could have no vlan tag or could have vlan tag.
      2. The port can be trunked, which let it carry multiple vlans over one EoMPLS tunnel
    2. Ethernet VLAN mode
      1. Is used to transport only one VLAN per pseudo wire over an EoMPLS tunnel
      2. The advantage is the ability to terminate VLANs on different remote PEs

Control Word on AToM

Control word is a 32-bit field that is inserted between inner MPLS label and the transported L2 frame.

Its purposes are:

  1. Pad small packets
  2. Carry control bits of Layer 2 header of the transported protocol (FECN, etc)
  3. Preserve the sequence of the transported frames (out of sequence packets are dropped)
  4. Facilitate the correct load balancing of AToM packets in the MPLS backbone
  5. Facilitate fragmentation and reassembly

You can think about Control Word as the 3rd label on a AToM packet. The 1st being the PE-to-PE label, the 2nd being the VC to VC label. Although Control Word is not really an MPLS label.

Control Word is optional for AToM (except for ATM AAL5 and FR). The use of Control Word is negotiated during the PW session setup between the PE.

Control Word is enable by default on Cisco IOS and IOS-XR

Control Word is REQUIRED for ATM AAL5 and Frame Relay

Some LSR forward traffic by looking into the 1st byte after the inner label to check whether the packet is a IPv4 or IPv6 packet when ECMP exists, when it found out the packet is an IP packet, it will try to load balanced the ECMP per flow by looking into the IP packet header. When Control Word is enable, the 1st byte is always 0, it helps the LSR not to accidentally treat a Layer 2 frame as IPv4 or IPv6 packets. For example, Ethernet header would start with 4, which is the same as IPv4 header.


Any Transport over MPLS is simple a method to transport L2 packets over MPLS network.

It uses two labels, the bottom label tells the Egress PE which Pseudo Wire that packet is part of. The top label define the tunnel between two PE.

LDP is used to distribute the tunnel labels (top labels)

Targeted LDP between with PE is used to distribute the pseudo wire label (bottom label).

As LSP is unidirectional, two pseudo wires, one for each direction is needed to have a duplex pseudo wire.

The pseudo wire signal protocol is called Label Mapping message. It runs over the target LDP between two PE.