H3C Technologies H3C S7500E Series Switches - Manual (Page 195)

5-3
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Outer label, also called tunnel label, is used to transfer packets from one PE to another.
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Inner label, also called VC label, is used to identify different connections between VPNs.
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Upon receiving packets, a PE determines to which CE the packets are to be forwarded according
to the VC labels.
Figure 5-2 illustrates how the label stack changes in the MPLS L2VPN forwarding process.
Figure 5-2 MPLS L2VPN label stack processing
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
MPLS L2VPN can be implemented in one of the following methods:
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Circuit Cross Connect (CCC) and Static Virtual Circuit (SVC): Two methods of implementing
MPLS L2VPN by configuring VC labels statically.
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Maritini: A method for establishing PPP links to implement MPLS L2VPN. It uses Label
Distribution Protocol (LDP) as a signaling protocol to transfer VC labels.
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Kompella: A CE-to-CE mode for implementing MPLS L2VPN on the MPLS network. It uses
extended BGP as the signaling protocol to advertise Layer 2 reachability information and VC
labels.
The following sections describe these implementation methods for MPLS L2VPN in detail.
CCC MPLS L2VPN
Unlike common MPLS L2VPN, Circuit Cross Connect (CCC) employs just one level of label to transfer
user data. Therefore, it uses label switched paths (LSPs) exclusively. That is, a CCC LSP can be used
to transfer only the data of the CCC connection; it can neither be used for other MPLS L2VPN
connections, nor for MPLS L3VPN or common IP packets.
The most significant advantage of this method is that no label signaling is required for transferring
Layer 2 VPN information. As long as MPLS forwarding is supported and service provider networks are
interconnected, this method works perfectly. In addition, since LSPs are dedicated, this method
supports QoS services.
There are two types of CCC connections: