MPLS-Enabled Applications: Emerging Developments and New Technologies

12.4. FORWARDING PLANE MECHANISMS

In this section, we discuss the forwarding mechanisms associated with VPLS in more detail. The user needs to configure which local ports are members of each VPLS on each PE. Each PE maintains a separate forwarding table for each VPLS. For example, PE1 in Figure 12.2 maintains a forwarding table for the VPLS associated with customer X and another forwarding table associated with the VPLS in customer Y.

A requirement of the VPLS scheme is that, for each VPLS, the participating PE routers should be fully meshed with pseudowires. This means that if a PE needs to send a frame to a remote site, it can send it directly to the appropriate remote PE without the frame having to be processed by an intermediary PE. The advantage of full meshing is that the PE routers do not have to run a spanning tree algorithm to eliminate the possibility of loops (in the same way that Ethernet switches need to run such an algorithm if they are not fully meshed).

We will first discuss how unicast Ethernet frames are forwarded to their destination and then discuss the treatment of broadcast and multicast frames.

12.4.1. Forwarding of unicast frames

Let us examine how unicast Ethernet frames are forwarded from the source to the destination. Referring again to Figure 12.2, let us focus on host A and host J in customer X's network. The MAC address of host A is A and the MAC address of host J is J. Let us suppose that host A sends a frame with source MAC address A to host J ...