There are two types of SDPs: spoke and mesh. The type of SDP defines how flooded traffic (or broadcast traffic, such as an ARP request) is propagated. For point-to-point PW/VLL services, spoke SDPs are the only way to bind services to the far-end router. For VPLS, mesh and spoke SDP bindings are allowed.
A spoke SDP that is bound to a service operates like a traditional bridge port. Flooded traffic that is received on the spoke SDP is transmitted to all the spoke SDPs, mesh SDPs, and SAPs to which it is connected. Flooded traffic is not transmitted back toward the port from which it was received.
In contrast, a mesh SDP that is bound to a service operates like a single bridge port. Flooded traffic received on a mesh SDP is transmitted to all spoke SDPs and SAPs to which it is connected. Flooded traffic is not transmitted to any other mesh SDPs or back toward the port from which it was received. This property of mesh SDPs is important for multi-node networks; mesh SDPs are used to prevent the creation of routing loops.