16
Figure 12 Network diagram for IGMP snooping proxying
As shown in
, Switch A works as an IGMP snooping proxy. As a host from the perspective of
the querier Router A, Switch A represents its attached hosts to send membership reports and leave
messages to Router A.
Table 7 IGMP message processing on an IGMP snooping proxy
IGMP message
Actions
General query
When receiving an IGMP general query, the proxy forwards it to all
ports but the receiving port. In addition, the proxy generates a report
according to the group memberships it maintains and sends the report
out all router ports.
Group-specific query
In response to the IGMP group-specific query for a certain multicast
group, the proxy sends the report to the group out all router ports if the
forwarding entry for the group still contains a member port.
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.
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.
Protocols and standards
•
RFC 4541,
Considerations for Internet Group Management Protocol (IGMP) and Multicast
Listener Discovery (MLD) Snooping Switches