MPLS-Enabled Applications: Emerging Developments and New Technologies

12.5. CONTROL PLANE MECHANISMS

In the previous section, we discussed the forwarding plane mechanisms for the VPLS. Let us now turn our attention to the control plane mechanisms. There are two aspects to be considered:

• The discovery aspect. How does a PE know which other PEs have members of a particular VPLS attached?

• The signaling aspect. How is a full mesh of pseudowires set up between those PEs?

For example, in Figure 12.2, PE1 has members of VPLS X and VPLS Y attached. It needs to know that PE2 and PE3 have members of VPLS X attached and that PE3 and PE4 have members of VPLS Y attached. It then needs a means to signal a pseudowire to PE2 and a pseudowire to PE3 pertaining to VPLS X and a pseudowire to PE3 and a pseudowire to PE4 pertaining to VPLS Y.

As with the Layer 2 point-to-point transport discussed in the previous chapter, there are two alternative schemes for the signaling aspect. One of the schemes is based on LDP and the other on BGP. The LDP scheme is very similar to the LDP scheme for point-to-point transport and the BGP scheme is very similar to the BGP scheme for point-to-point transport discussed in the previous chapter. With regard to the discovery aspect, the BGP scheme has a built-in automated mechanisms for this discovery process (as is the case with L2 point-to-point transport signaled using BGP), so the process is known as autodiscovery. In contrast, the LDP scheme does not support in-built autodiscovery. Therefore in the LDP case, either one must manually ...