MPLS-Enabled Applications: Emerging Developments and New Technologies

11.9. LAYER 2 INTERWORKING

The Layer 2 schemes described so far require both ends of a Layer 2 transport connection, or all the tail circuits of a given Layer 2 VPN, to be of the same Layer 2 media type. This can be a constraint in situations where a customer uses more than one media type, perhaps as a consequence of the prevalence of different media types in the various regions in which the customer is located, or as a consequence of mergers and acquisitions or because the customer is in the middle of migrating from one type of access medium to another. One way of relaxing this constraint is Layer 2 interworking, also known as 'Layer 2.5 VPNs'. This allows different media types to be used as the tails of the Layer 2 connections, with the proviso that the packets being transported over the connection must be IP packets.

In this scheme, when a packet arrives at a PE from the local CE, the entire Layer 2 encapsulation is stripped off, exposing the underlying IP packet. Note this is unlike the treatment of the Layer 2 frames described in Section 11.5 of this chapter, in which certain parts of the Layer 2 header are retained for transport to the remote PE. The underlying IP packet has the VPN label applied plus any transport labels and is sent to the remote PE. The remote PE extracts the IP packet and applies the appropriate Layer 2 encapsulation corresponding to the local Layer 2 tail circuit.

Only IP packets can be transported using this scheme because there is no way for the receiving ...