3com WX3000 Manuel D’Utilisation

 

2-3 

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 device forwards it through all ports in the VLAN except the 

receiving port and performs the following to the receiving port:  

z 

If the receiving port is a router port existing in its router port list, the device resets the aging timer of 

this router port.  

z 

If the receiving port is not a router port existing in its router port list, the device adds it into its router 

port list and sets an aging timer for this router port.  

A host sends an IGMP report to the multicast router in the following circumstances:  

z 

Upon receiving an IGMP query, a multicast group member host responds with an IGMP report.  

z 

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 device 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:  

z 

If the port is already in the forwarding table, the device resets the member port aging timer of the 

port.  

z 

If the port is not in the forwarding table, the device installs an entry for this port in the forwarding 

table and starts the member port aging timer of this port.  

 

A device will not forward an IGMP report through a non-router port for the following reason: Due to the 

IGMP report suppression mechanism, if member hosts of that multicast group still exist under 

non-router ports, the hosts will stop sending reports when they receive the message, and this prevents 

the device from knowing if members of that multicast group are still attached to these ports.  

 

When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leave message, so 

the device cannot know immediately that the host has left the multicast group. However, as the host 

stops sending IGMP reports as soon as it leaves a multicast group, the device deletes the forwarding 

entry for the member port corresponding to the host from the forwarding table when its aging timer 

expires.  

When an IGMPv2 or IGMPv3 host leaves a multicast group, the host sends an IGMP leave message to 

the multicast router to announce that it has leaf the multicast group.  

Upon receiving an IGMP leave message on the last member port, a device forwards it out all router 

ports in the VLAN. Because the device does not know whether any other member hosts of that multicast 

group still exists under the port to which the IGMP leave message arrived, the device does not