MPLS Layer 2 VPNs


  • Offers complete separation between provider and customer networks since there is no route exchange.

  • This is the same approach as overlay models (e.g. FR, ATM).

  • Carrier customer's layer 2 frames from one site to another transparently, and is hence independent of layer 3 protocols:

    • can carry IPv4, IPv6, IPX, DECnet, etc.

  • Can provide:

    • point-to-point connectivity.

    • multi-point connectivity.


  • Martini draft uses "VC" concept:

    • an LSP can carry multiple VCs.

    • a VC acts like a circuit carrying customer layer 2 frames.

  • A VC is an LSP within the original tunnel LSP:

    • the tunnel LSP provides the tunnel between two PE routers.

    • a VC carries frames for a given customer only.

    • VCs are uni-directional, so a pair is needed for bi-directional communications.

    • the customer frame is encapsulated in two labels:

L2 Header – Label 1 – Label 2 – Optional Control Word – Tagged / Untagged Ethernet Payload

Label 1 is for the tunnel LSP

Label 2 is for the VC


  • Frames are sent between multiple customer sites within a VPN:

    • sent only to the PE that connects to the target site (based on destination MAC).

    • VPLS: Virtual Private LAN Services (draft Kompella).

  • Builds full mesh of VCs (x2 because of uni-directionality of VCs) between member PEs in a VPN:

    • VPLS uses LDP (* see below) to exchange VC labels between PE routers.

  • VPN ID = 32 bits (?)

  • PE routers learn MAC addresses just like normal switches, but only on received frames over VCs:

    • if CE → PE frame destination MAC is not known, then the frame is flooded over all VCs in that VPN.

  • PE routers maintain layer 2 forwading table: VFI = Virtual Forwarding Instance.

  • PE routers do not have to run STP because of split horizon:

    • customer frames received on a VCs can only be forwarded to an attached customer CE router (i.e. not back over any VC).

VC Label Exchange Between PE Routers

  • Juniper M / T / J series: BGP

  • Juniper E series: LDP

  • Cisco: LDP

  • Alcatel-Lucent: LDP

  • Ericsson / Redback: LDP

BGP: draft Kireeti Kompella (Juniper

LDP: draft Vach Kompella (Alcatel-Lucent)

© Robert Larsen. All rights reserved.