ToM Architecture

In networks that use AToM, all the routers in the service provider network run MPLS, and the PE routers have an AC toward the CE router. The PE router receives Layer 2 frames on the AC and encapsulates them with labels before sending them onto the PSN tunnel toward the remote PE. At the remote PE, the label(s) are removed and the frames are sent toward the remote CE.

In the case of AToM, the PSN tunnel is nothing other than a label switched path (LSP) between two PE routers. As such, the label that is associated with that LSP is called the tunnel label in the context of AToM. The LSRs can signal that LSP in different ways. First, the Label Distribution Protocol (LDP) can signal the LSP hop-by-hop between the two PEs. Second, the LSP can be an MPLS traffic engineering (TE) tunnel that the Resource Reservation Protocol (RSVP) signals with the extensions needed for TE. With this tunnel label, you can identify to which PSN tunnel the carried customer frame belongs. This tunnel label also gets the frames from the local or ingress PE to the remote or egress PE across the MPLS backbone. To multiplex several pseudowires onto one PSN tunnel, the PE router uses another label to identify the pseudowire. This label is called the VC or PW label because it identifies the virtual circuit or the pseudowire that the frame is multiplexed onto. Figure 10-2 shows the pseudowire emulation edge-to-edge reference model, but translated to a network where the PSN is an MPLS network.

Figure 10-2 Pseudowire Emulation Edge-to-Edge Reference Model for an MPLS Network

Pseudowire 1 Pseudowire n

CE PE

Attachment LSR LSR LSR

Circuit (AC)

Emulated Tunnel LSP

Signaled by LDP or RSVP

Figure 10-2 Pseudowire Emulation Edge-to-Edge Reference Model for an MPLS Network

Signaled by LDP or RSVP

Pseudowire 1 Pseudowire n

LSR Attachment Circuit (AC)

LSR Attachment Circuit (AC)

An LSP is unidirectional. Therefore, for a pseudowire to be set up, two LSPs must exist between a pair of PE routers, one for each direction.

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