H3C Technologies H3C S12500 Series Switches - Manual (Page 33)

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Figure 13 Network diagram
As shown in Figure 13 , Switch A operates 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 6 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 except the port
that receive the query. In addition, the proxy generates a report according to the group
membership that it maintains and sends the report out of 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 of all router ports if the forwarding entry for the group still
contains a member port.
Report
After receiving a report for a multicast group, the proxy looks up the multicast forwarding
table for the forwarding entry for the multicast group.
• If a forwarding entry matches the multicast group and contains the receiving port as a
dynamic member port, the proxy restarts the aging timer for the port.
• If a forwarding entry matches the multicast group but does not contain the receiving
port, the proxy adds the port to the forwarding entry as a dynamic member port and
starts an aging timer for the port.
• If no forwarding entry matches the multicast group, the proxy creates a forwarding
entry for the multicast group, adds the receiving port to the forwarding entry as a
dynamic member port, and starts an aging timer for the port.
Leave
In response to an IGMP leave message for a multicast group, the proxy sends a
group-specific query out of 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 of all router ports.
Protocols and standards
RFC 4541, Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener
Discovery (MLD) Snooping Switches
Host A
Receiver
Host B
Host C
Receiver
IGMP Querier
Router A
Proxy & Querier
Switch A
Query from Router A
Report from Switch A
Query from Switch A
Report from Host
IP network