Transporting Layer 2 Frames

Two solutions are available for transporting Layer 2 frames across a packet-switched network (PSN):

■ Carry the traffic across an MPLS backbone, which is the AToM solution.

■ Carry the traffic across an IP backbone, which is the Layer 2 Tunneling Protocol version 3 (L2TPv3) solution.

NOTE Both solutions are implemented in Cisco IOS, but because this book focuses on MPLS, only the AToM solution is explained here.

L2TPv3 is the Layer 2 transport service over an IP network. The Layer 2 frames are encapsulated with an L2TPv3 header and are transported across the IP network. Similar to AToM, L2TPv3 can carry ATM, Frame Relay, HDLC, PPP, Ethernet, ATM, and others.

Both the AToM and L2TPv3 solutions use the same architecture, but the network that is carrying the service is different. The architecture is based on pseudowires. The pseudowires carry the customer Layer 2 traffic from edge to edge across the packet-switched backbone network, whether it is an MPLS or an IP-based backbone. The pseudowire is a connection between the PE routers and emulates a wire that is carrying Layer 2 frames. Pseudowires use tunneling. The Layer 2 frames are either encapsulated into an IP packet (L2TPv3) or are labeled (MPLS). The result is that the specific Layer 2 service—its operation and characteristics—is emulated across a PSN. The Internet Engineering Task Force (IETF) specifies the architecture and encapsulation in several RFCs and drafts. In Figure 10-1, you can see the pseudowire emulation edge-to-edge reference model, as specified by the IETF.

Figure 10-1 Pseudowire Emulation Edge-to-Edge Reference Model

PSN Tunnel

Pseudowire n

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