Layer-2 VPNs


Overview

  • 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.

Point-to-Point

  • 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:
Layer 2
Header
Label 1
(for the tunnel LSP)
Label 2
(for the VC)
 Optional
Control Word
 Tagged or untagged
Ethernet payload

Multi-Point

  • 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 BGP for M/T/J series
LDP for E series 
 Cisco LDP 
 Alcatel-Lucent LDP 
 Ericsson/Redback LDP 
  • BGP: draft Kireeti Kompella (Juniper)
  • LDP: draft Vach Kompella (Alcatel-Lucent)
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Robert Larsen,
19 Jan 2010, 18:35
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MPLSLayer2VPNs.vsd
(1644k)
Robert Larsen,
19 Jan 2010, 18:35