Protocols And Standards; Recommended Configuration Procedure - HP FlexNetwork NJ5000 User Manual

An IGMPv2 or IGMPv3 host sends an IGMP leave message to the multicast router when it leaves a
multicast group.
When the switch receives an IGMP leave message on a dynamic member port, the switch first
examines whether a forwarding entry matches the group address in the message, and, if a match is
found, whether the forwarding entry for the group contains the dynamic member port.
•
If no forwarding entry matches the group address, or if the forwarding entry does not contain the
port, the switch directly discards the IGMP leave message.
•
If a forwarding entry matches the group address and the forwarding entry contains the port, the
switch forwards the leave message to all router ports in the VLAN. Because the switch does not
know whether any other hosts attached to the port are still listening to that group address, the
switch does not immediately remove the port from the forwarding entry for that group. Instead, it
restarts the aging timer for the port.
After receiving the IGMP leave message, the IGMP querier resolves the multicast group address in
the message and sends an IGMP group-specific query to the multicast group through the port that
received the leave message. After receiving the IGMP group-specific query, the switch forwards it
through all its router ports in the VLAN and all member ports of the multicast group. The switch also
performs one of the following actions for the port that received the IGMP leave message:
•
If the port (assuming that it is a dynamic member port) receives an IGMP report in response to
the group-specific query before its aging timer expires, it means that some host attached to the
port is receiving or expecting to receive multicast data for the multicast group. The switch
restarts the aging timer for the port.
•
If the port receives no IGMP report in response to the group-specific query before its aging timer
expires, it means that no hosts attached to the port are still listening to that group address. The
switch removes the port from the forwarding entry for the multicast group when the aging timer
expires.

Protocols and standards

RFC 4541, Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener
Discovery (MLD) Snooping Switches

Recommended configuration procedure

Step
Enabling IGMP
1.
snooping globally
Enabling dropping
2.
unknown multicast
data globally
Configuring IGMP
3.
snooping in a VLAN
Remarks
Required.
Disabled by default.
Optional.
Unknown multicast data refers to multicast data for which no forwarding entries
exist in the forwarding table. When the switch receives such multicast traffic,
one of the following situations occurs:
•
If dropping unknown multicast data is disabled, the switch floods unknown
multicast data in the VLAN.
•
If dropping unknown multicast data is enabled, the switch drops all
received unknown multicast data.
Disabled by default.
Enable IGMP snooping globally before you enable dropping unknown multicast
data globally.
Required.
Enable IGMP snooping in the VLAN and configure the IGMP snooping version
and querier feature.
By default, IGMP snooping is disabled in a VLAN.
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