Work Mechanism Of Igmp Snooping - H3C S3100 8C SI Operation Manual

Operation Manual – Multicast
H3C S3100 Series Ethernet Switches
Timer
Member port
aging timer

2.1.3 Work Mechanism of IGMP Snooping

A switch running IGMP Snooping performs different actions when it receives different
IGMP messages, as follows:
I. When receiving a general query
The IGMP querier periodically sends IGMP general queries to all hosts and routers on
the local subnet to find out whether active multicast group members exist on the
subnet.
Upon receiving an IGMP general query, the switch forwards it through all ports in the
VLAN except the receiving port and performs the following to the receiving port:
If the receiving port is a router port existing in its router port list, the switch resets
the aging timer of this router port.
If the receiving port is not a router port existing in its router port list, the switch adds
it into its router port list and sets an aging timer for this router port.
II. When receiving a membership report
A host sends an IGMP report to the multicast router in the following circumstances:
Upon receiving an IGMP query, a multicast group member host responds with an
IGMP report.
When intended to join a multicast group, a host sends an IGMP report to the
multicast router to announce that it is interested in the multicast information
addressed to that group.
Upon receiving an IGMP report, the switch forwards it through all the router ports in the
VLAN, resolves the address of the multicast group the host is interested in, and
performs the following to the receiving port:
If the port is already in the forwarding table, the switch resets the member port
aging timer of the port.
If the port is not in the forwarding table, the switch installs an entry for this port in
the forwarding table and starts the member port aging timer of this port.
Description
When a port joins a
multicast group, the
switch sets a timer for
the port, which is
initialized to the member
port aging time
2-3
Chapter 2 IGMP Snooping Configuration
Message
before expiry
The switch
IGMP removes this port
membership from the multicast
report group forwarding
table
Action after
expiry