3Com 4800G Series, Configuration Manual

Page: 557 / 1246

IGMP Snooping Overview 557
■
If the forwarding table entry exists and its outgoing port list contains the port,
the switch forwards the leave group 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
removes the port from the outgoing port list of the forwarding table entry for
that group; instead, it resets the member port aging timer for the port.
Upon receiving the IGMP leave group message from a host, the IGMP querier
resolves from the message the address of the multicast group that the host just
left and sends an IGMP group-specific query to that multicast group through the
port that received the leave group message. Upon hearing the IGMP
group-specific query, the switch forwards it through all its router ports in the VLAN
and all member ports for that multicast group, and performs the following:
■
If any IGMP report in response to the group-specific query is heard on a
member port before its aging timer expires, this means that some host
attached to the port is receiving or expecting to receive multicast data for that
multicast group. The switch resets the aging timer of the member port.
■
If no IGMP report in response to the group-specific query is heard on a member
port before its aging timer expires, this means that no hosts attached to the
port are still listening to that group address: the switch removes the port from
the outgoing port list of the forwarding table entry for that multicast group
when the aging timer expires.
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:
■
The switch broadcasts IGMP messages as unknown messages in the VLAN.
■
Upon receiving a PIM hello message, the switch will maintain the
corresponding router port.
3 When IGMP is disabled on the switch, or when IGMP forwarding entries are
cleared (by using the
reset igmp group command):
■
If PIM is disabled, the switch clears all its Layer 2 multicast entries and router
ports.
■
If PIM is enabled, the switch clears only its Layer 2 multicast entries without
deleting its router ports.
4 When PIM is disabled on the switch:
■
If IGMP is disabled, the switch clears all its router ports.
■
If IGMP is enabled, the switch maintains all its Layer 2 multicast entries and
router ports.
Protocols and Standards IGMP Snooping is documented in:
■
RFC 4541: Considerations for Internet Group Management Protocol (IGMP)
and Multicast Listener Discovery (MLD) Snooping Switches