3Com 10014298 manual Specific group query

Configuring IGMP 117

discover whether hosts join the specified group on its subnets according to the received response messages. When the router receives the report that hosts leave the group, the router will send a group-specific query (IGMP Version 2) to discover whether there are no members in the group.

IGMP has three versions, IGMP Version 1 (defined by RFC1112), IGMP Version 2 (defined by RFC2236) and IGMP Version 3. IGMP Version 2 is the most widely used version.

IGMP Version 2 boasts the following improvements over IGMP Version 1:

■Election mechanism of multicast routers on the shared network segment

A shared network segment means that there are multiple multicast routers on a network segment. In this case, all routers running IGMP on the network segment can receive the membership report from hosts. Therefore, only one router is required to send membership query messages. In this case, the router election mechanism is required to specify a router as the querier.

In IGMP Version 1, selection of the querier is determined by the multicast routing protocol. IGMP Version 2 specifies that the multicast router with the lowest IP address is elected as the querier when there are multiple multicast routers on the same network segment.

■Leaving group mechanism

In IGMP Version 1, hosts leave the multicast group quietly without informing the multicast router. The multicast router can only depend on the timeout of the response time to confirm when hosts leave the group. In Version 2, when a host leaves a multicast group, it will send a leave group message.

■Specific group query

In IGMP Version 1, a query of multicast routers is targeted at all the multicast groups on the network segment. This is known as General Query.

In IGMP Version 2, besides general query, Group-Specific Query is added. The destination IP address of the query packet is the IP address of the multicast group. The group address domain in the packet is also the IP address of the multicast group. This prevents the hosts of members of other multicast groups from sending response messages.

■Max response time

The Max Response Time was added in IGMP Version 2. It is used to dynamically adjust the allowed maximum time for a host to respond to the membership query message.

Configuring IGMP Once multicast is enabled, IGMP will automatically run on each interface. Generally, IGMP does not need to be configured. In the following configuration, only the first one is mandatory.

Basic IGMP configuration includes:

■Enabling Multicast

■Enabling IGMP on an Interface