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IGMP message
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Report
When receiving a report for a multicast group, the proxy looks up the multicast
forwarding table for the entry for the multicast group. If the forwarding entry is
found with the receiving port contained as a dynamic member port in the outgoing
port list, the proxy resets the aging timer for the entry. If the forwarding entry is
found but the outgoing port list does not include the receiving port, the proxy adds
the port to the outgoing port list as a dynamic member port and starts an aging
timer for it. If no forwarding entry is found, the proxy creates the entry, adds the
receiving port to the outgoing port list as a dynamic member port and starts an
aging timer for the port, and then, sends a report to the group out all router ports.
forwarding table for the entry for the multicast group. If the forwarding entry is
found with the receiving port contained as a dynamic member port in the outgoing
port list, the proxy resets the aging timer for the entry. If the forwarding entry is
found but the outgoing port list does not include the receiving port, the proxy adds
the port to the outgoing port list as a dynamic member port and starts an aging
timer for it. If no forwarding entry is found, the proxy creates the entry, adds the
receiving port to the outgoing port list as a dynamic member port and starts an
aging timer for the port, and then, sends a report to the group out all router ports.
Leave
In response to an IGMP leave message for a multicast group, the proxy sends a
group-specific query out the receiving port. After making sure that no member
port is contained in the forwarding entry for the multicast group, the proxy sends a
leave message to the group out all router ports.
group-specific query out the receiving port. After making sure that no member
port is contained in the forwarding entry for the multicast group, the proxy sends a
leave message to the group out all router ports.
Processing of Multicast Protocol Messages
With Layer 3 multicast routing enabled, an IGMP snooping switch processes multicast protocol
messages differently under different conditions, specifically as follows:
1) If only IGMP is enabled, or both IGMP and PIM are enabled on the switch, the switch handles
multicast protocol messages in the normal way.
2) In only PIM is enabled on the switch:
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The switch broadcasts IGMP messages as unknown messages in the VLAN.
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Upon receiving a PIM hello message, the switch will maintain the corresponding dynamic router
port.
3) When IGMP is disabled on the switch:
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If PIM is disabled, the switch deletes all its dynamic member ports and dynamic router ports.
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If PIM is enabled, the switch deletes only its dynamic member ports without deleting its dynamic
router ports.
On a switch with Layer-3 multicast routing enabled, use the display igmp group port-info command to
view Layer-2 port information.
For details about the display igmp group port-info command, refer to IGMP Commands in the IP
Multicast Volume.
4) When PIM is disabled on the switch:
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If IGMP is disabled, the switch deletes all its dynamic router ports.
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If IGMP is enabled, the switch maintains all its dynamic member ports and dynamic router ports.
Protocols and Standards
IGMP snooping is documented in:
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RFC 4541: Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener
Discovery (MLD) Snooping Switches