The setup, maintenance, and teardown of pseudowires forming a VPLS can be achieved using either an LDP or BGP control plane, although the functional behavior of VPLS is the same across both models. The LDP control plane model deals exclusively with the pseudowire signaling aspects, while the BGP model (RFC 4761) is broken into the sub-components of Auto-Discovery and pseudowire signaling; both can be fulfilled within a single BGP UPDATE message.
The Auto-Discovery and signaling sub-components are enabled independently in SR-OS, which leads to support of a further “hybrid” model where Auto-Discovery is managed by BGP but pseudowire signaling is managed by LDP. It's even possible that within a single VPLS instance, some parts of the service use LDP signaling and some use BGP signaling (for example, a Hierarchical-VPLS implementation with a core using BGP, and metro areas using LDP).
To identify other PE routers forming part of the VPLS (as part of the Auto-Discovery mechanism) the Route Target Extended Community (RFC4360) is used. The semantics of the use of Route Targets are exactly the same as used for BGP-MPLS IP-VPN. If a VPLS is fully meshed, a single Route Target suffices. A VPLS NLRI is also introduced with L2VPN AFI (25) and VPLS SAFI (65) for the purpose of declaring VPLS membership and exchanging demultiplexors.
To declare VPLS membership, a PE router belonging to a given VPLS announces its VPLS NLRI with the relevant Route-Target and accepts ...