The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

12.5. Multicast Registration and VLAN Context

It is possible (in fact, likely) that a VLAN-aware device implementing GVRP will also be capable of multicast pruning and will include GMRP as part of its operation. As discussed in Chapter 10, "Multicast Pruning," GMRP allows switches to restrict multicast propagation to a proper subset of the spanning tree. For a catenet comprising both GMRP- and VLAN-aware devices, 802.1Q extends GMRP capability to support separate multicast registrations within each VLAN. That is, rather than just restricting multicast propagation relative to the catenet as a whole, a combined GMRP/VLAN-aware device can restrict multicasts to a subset of a VLAN (which is already a subset of the catenet). This permits more fine-grained control than would otherwise be possible.