- Enterasys Security Router User's Guide

Describing the XSR’s IP Multicast Features
XSR User’s Guide 7-5
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-group command 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-time command. 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-interval command. 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.