C H A P T E R 1 2
IP Multicast Routing
229
IGMP Overview
IGMP is a protocol used by an IP host to register its IP multicast
group membership with a router. The messaging protocol can also be
“snooped” by a layer 2 switch, to provide for intelligent forwarding
of multicast data streams within a VLAN. Periodically, the router
queries the multicast group to see if the group is still in use. If the
group is still active, a single IP host responds to the query, and group
registration is maintained.
IGMP Snooping
IGMP snooping is a layer 2 function of the switch. The feature
reduces the flooding of IP multicast traffic, optimizes the usage of
network bandwidth, and prevents multicast traffic from being flooded
to parts of the network that do not need it. The switch does not reduce
any IP multicast traffic in the local multicast domain (224.0.0.x). An
optional optimization for IGMP snooping is the strict recognition of
multicast routers only if the remote devices have joined the DVMRP
(224.0.0.4) or PIM (244.0.0.13) multicast groups.
IGMP snooping is enabled by default on the switch. If IGMP
snooping is disabled, all IGMP and IP multicast traffic floods within
a given VLAN. This is standard 802.1d bridge behavior. IGMP
snooping expects to see periodic IGMP reports from interested hosts
on each port. Without an IGMP querier, the switch may stop
forwarding IP multicast packets to all ports.
To support IGMP snooping in environments that do not have an
IGMP querier, the switch can function as an IGMP querier, according
to the rules of standard IGMP Version 2.0. If IGMP snooping is
enabled, the switch periodically queries for multicast group
memberships. However, if either IGMP snooping is disabled or
IGMP functionality is disabled, the switch does not generate IGMP
query messages. IGMP is enabled when the switch is configured to
perform IGMP snooping and there is no other reliable querier on the
network.
IGMP configuration commands are described in Table 58.
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Страница 16: ...14 P R E F A C E...
Страница 88: ...86 C H A P T E R 4 Configuring Switch Ports...
Страница 112: ...110 C H A P T E R 5 Virtual LANs VLANs...
Страница 152: ...150 C H A P T E R 8 Quality of Service QoS...
Страница 166: ...164 C H A P T E R 9 Enterprise Standby Router Protocol...
Страница 198: ...196 C H A P T E R 1 0 IP Unicast Routing...
Страница 228: ...226 C H A P T E R 1 1 RIP and OSPF...
Страница 254: ...252 C H A P T E R 1 3 IPX Routing...
Страница 274: ...272 C H A P T E R 1 4 Access Policies...
Страница 296: ...294 C H A P T E R 1 6 Using Web Device Manager...
Страница 320: ...318 A P P E N D I X A...
Страница 328: ...326 A P P E N D I X B...
Страница 346: ...344 A P P E N D I X C...
Страница 358: ...356 I N D E X...
Страница 366: ...364 I N D E X...