3Com MSR 50 Series Configuration Manual page 1427

Provider edge router (PE): A PE resides on a service provider network and
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connects one or more CEs to the network. On an MPLS network, all VPN
processing occurs on the PEs.
Provider (P) router: A P router is a backbone router on a service provider
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network. It is not directly connected with any CE. It only needs to be equipped
with basic MPLS forwarding capability.
MPLS L2VPN uses label stacks to implement the transparent transmission of user
packets in the MPLS network.
Outer label, also called tunnel label, is used to transfer packets from one PE to
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another.
Inner label, also called VC label, is used to identify different connections
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between VPNs.
Upon receiving packets, a PE determines to which CE the packets are to be
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forwarded according to the VC labels.
Figure 398 illustrates how the label stack changes in the MPLS L2VPN forwarding
process.
Figure 398 MPLS L2VPN label stack processing
T V L2PDU
PE 1
L2PDU
CE 1
1) L2 PDU: Layer 2 protocol data unit
2) T represents tunnel label. V represents VC label. T' represents swapped tunnel label.
Implementation of MPLS L2VPN
Currently, there is no official standard for MPLS L2VPN. The provider-provisioned
virtual private network (PPVPN) working group of the IETF has drafted several
framework protocols. Two of the most important ones are Martini draft and
Kompella draft:
draft-martini-l2circuit-trans-mpls
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draft-kompella-ppvpn-l2vpn
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The Martini draft defines a method for establishing PPP links to implement MPLS
L2VPN. It uses label distribution protocol (LDP) as a signaling protocol to transfer
VC labels.
P
T` V L2PDU
PE 2
L2PDU
CE 2
MPLS L2VPN Overview 1427