In addition,
if a bridge receives a registration for an additional VLAN from a neighbor bridge, then
it registers that VLAN with all of the other bridges (but not back toward that neighbor
bridge, unless some other bridge needs it). Whenever the spanning tree topology
changes, MVRP immediately reregisters the VLANs.
Given this VLAN registration information, each bridge creates one-way filters that
prevent any traffic for a given VLAN from being transmitted on a port, unless a neighbor
attached to that port has registered that it needs to receive that VLAN. Thus,
frames on a particular VLAN are never forwarded toward those parts of the network
where they are not needed.
Ethernet Bridging 383
Although there was a similar protocol, the Generic Multicast Registration Protocol
(GMRP), for registering multicast MAC addresses, it did not gain acceptance in the
market. Instead, bridges typically ???snoop??? on IGMP packets, and may even intercept,
summarize, and reissue IGMP packets themselves, as if they were routers. Although
this violates the principles of proper layering, it has proven to be a cost-effective solution
to the problem of distributing multicast streams, which can take up considerable bandwidth,
to parts of the bridged network where they are not needed.
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