RuggedCom RS400 manual Switch Igmp Operation, Active Mode

Multicast Filtering

Membership Query

M2 Membership Report

		P1
M2			M1
Multicast
	Router

Membership Query

M1 Membership Report

C1 C2 C3 C4

Figure 135: IGMP Operation Example 1

In this example the general membership query sent to the C1-C2 segment is answered by a membership report indicating the desire to subscribe to a stream M2. The router will forward the M2 stream onto the C1-C2 segment. In a similar fashion the router discovers that it must forward M1 onto segment C3-C4.

Note: Membership reports are also referred to as “joins”.

A consumer may join any number of multicast groups, issuing a membership report for each group. Hosts on the segment note membership reports from other hosts and will suppress their own reports accordingly. In this way the IGMP protocol guarantees the segment will issue only one join for each group.

The router periodically queries each of its segments, in order to determine if at least one consumer still subscribes to a given stream. If no responses occur within a given timeout period (usually about two query intervals) the router will prune the multicast stream from the given segment.

A more usual method of pruning occurs when consumers wishing to unsubscribe issue an IGMP “leave group” message. The router will immediately issue a group-specific membership query to determine whether there are any remaining subscribers of that group on the segment. After the last consumer of a group has un-subscribed, the router will prune the multicast stream from the given segment.

8.1.2 Switch IGMP Operation

The IGMP Snooping feature provides a means for switches to snoop (i.e. watch) the operation of routers, respond with joins/leaves on the behalf of consumer ports and to prune multicast streams accordingly.

There are two modes of IGMP the switch can be configured to assume, active and passive.

Active Mode

ROS™ IGMP supports “routerless” mode of operation.

When such a switch is used without a multicast router, it is able to function as if it is a multicast router sending IGMP general queries.

ROS™ v3.5 196 RS400