Enterasys Networks X-PeditionTM manual Group Membership Actions, Sending a Query

Describing the XSR’s IP Multicast Features

IGMP is an asymmetric protocol, so there are separate behaviors for group members (hosts or routers that wish to receive multicast packets) and multicast routers (routers that can forward multicast packets).

Group Membership Actions

Group members transmit Report messages to inform neighboring multicast routers of their multicast group states. Two types of events can trigger IGMPv3 protocol actions on an interface, a change in the interface reception state, and a query reception.

•A port reception state change is usually triggered by a higher-layer multicast program for video conferencing or network meetings. When you reconfigure these applications, that may change the multicast state triggering the system to send a state-change report from the associated interface. The type and contents of the group records in that report are determined by comparing the filter mode and source list for the affected multicast address before and after the change. The ip igmp join-groupcommand simulates a host to join a specific multicast group.

•When the XSR receives a Query, it delays its response by a random interval - the max resp time value - derived from the max resp code in the received Query message. You can set this value in the sending router with the ip igmp query-max-response-timecommand. An XSR may receive a variety of Queries on different interfaces including General Queries, Group-Specific Queries, and Group-and-Source-Specific Queries, each of which may require its own delayed response. Before scheduling a response to a Query, the XSR must first consider previously scheduled pending responses and in many cases schedule a combined response.

Sending and Receiving Queries and Reports

Multicast routers send Query messages to and receive Report messages from group members. Multicast routers need know only that at least one system on an attached network is interested in packets to a particular multicast address from a source. The multicast router is not required to keep track of the interests of each individual neighboring system.

Sending a Query

Multicast routers periodically send General Queries to request group membership data from an attached network, a value which you can set with the ip igmp query-intervalcommand. These queries help build and refresh the group membership state of attached systems which respond by reporting that state in IGMPv3 Membership Reports.

Multicast routers also transmit specific queries enabling all network systems to respond to group membership changes. Group-Specific Queries are sent to verify no systems want to receive the specified group or rebuild the desired reception state for a particular group. They are sent when a router gets a State-Change record indicating a system withdrawal from the group.

A Group-and-Source Specific Query verifies no network systems want traffic from a set of sources. It lists sources for a particular group which have been requested to no longer be forwarded. This query is sent by a multicast router to learn if any systems want to receive packets to the specified group address from the specified source addresses. These queries are sent only in response to State-Change Records, never in response to Current-State Records.

XSR User’s Guide 7-5