IGMP Snooping
IGMP snooping prevents multicast flows from flooding to all
ports on a VLAN by monitoring the Layer 3 IGMP packets. Multicast streams are
sent to ports that explicitly request the flow. The switch via the IGMP snooping
mechanism listens to the conversation between the router and the host machine.
The switch learns at Layer 3 which port is signaling for or leaving a multicast
group. The switching engine forwards this message to the Network Management
Processor (NMP), where the port is added to or removed from the Layer 2
multicast forwarding group based on the IGMP message type.
The first step that is required for IGMP snooping is that the
switch needs to learn the router port. Typically, a Protocol Independent
Multicast (PIM) hello message signals the switch where the router port is
located. The following messages are used for locating the router port:
-
IGMP Group Membership Queries (01-00-5e-00-00-01 or
224.0.0.1)
-
PIM v1 Queries (01-00-5e-00-00-02 or 224.0.0.2)
-
PIM v2 Queries (01-00-5e-00-00-0d or 224.0.0.13)
-
DVMRP messages (01-00-5e-00-00-04 or 224.0.0.4)
-
MOSPF messages (01-00-5e-00-00-05 or 224.0.0.5 and
01-00-5e-00-00-06 or 224.0.0.6)
Figure 9-4 shows that
the source and receivers are directly connected to the same switch. The switch
has a multilayer switch feature card (MSFC), which will handle inter-VLAN
communication. Therefore, the MSFC, port 15/1, will be the router port.
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