When the loss or recovery of a link or a bridge changes the spanning
tree, many MAC addresses are forgotten and the flooding of unknown addresses can
saturate a network??™s physical carrying capacity. Furthermore, ordinary amounts of
broadcast and multicast traffic, if flooded everywhere, could waste considerable bandwidth
on an ongoing basis.
Bridged networks avoid this problem by pruning both VLANs and multicast MAC
addresses. VLANs are pruned using the Multiple VLAN Registration Protocol (MVRP,
IEEE 802.1ak), recently completed. Multicast MAC addresses are pruned by means of
the bridges ???snooping??? on the same Layer 3 Internet Group Multicast Protocol (IGMP)
used to regulate the IP multicasts that carry most high-volume multicast traffic.
In a nutshell, MVRP allows each bridge to register with each of its neighbors which
VLANs??™ frames it needs to have delivered to it. A bridge registers with its neighbors
all of the VLANs for which it has ports configured. For example, if a bridge has two
untagged ports, A and B, configured to default incoming frames to VLANs 5 and 7,
respectively, then it will register VLANs 5 and 7 with its neighbor bridges.
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