MPLS-Enabled Applications: Emerging Developments and New Technologies

11.5. FORWARDING PLANE

This section describes the operation of Layer 2 transport in the forwarding plane. Note that the encapsulation method is the same regardless of whether the BGP or LDP control plane scheme is in use. The detail of how Layer 2 packets are encapsulated for transport over an MPLS network in a pseudowire is contained in an IETF document entitled 'Encapsulation methods for transport of Layer 2 frames over IP and MPLS networks' [MRT-ENC]. This is now a Historic RFC, having been superseded by a series of documents, one per media type, produced by the Pseudowire Emulation Edge-to-Edge (PWE3) Working Group in the IETF. The PWE3 documents contain more encapsulation variants than [MRT-ENC] for some Layer 2 media, but most existing implementations are closer to [MRT-ENC].

Figure 11.3 shows a cross-section through the network shown in Figure 11.2, showing the transport of Layer 2 packets on a pseudowire between CE1 and CE2. When the Layer 2 frame arrives at PE1, PE1 carries out the following forwarding operations:

Figure 11.3. Forwarding plane operation of L2 transport over MPLS

1. 1. Parts of the L2 frame that do not need to be transported to the remote PE are removed. For example, in the Ethernet case the FCS (Frame Check Sequence) is removed.

2. 2. In some cases, a 4-byte Control Word (CW) is prepended to the L2 frame. The Control Word can include a sequence number so that ...