User Manual

100 AR400 Series Router User Guide
Software Release 2.6.1
C613-02021-00 REV D
While you can configure different multicasting protocols on different interfaces
on the same router, multicasting information is not translated between the
different multicast protocols.
Configuring IGMP
By default, IGMP is disabled on the router and on all interfaces. To enable
IGMP on the router, enter the command:
ENABLE IP IGMP
You must enable IGMP on an interface before the interface can send or receive
IGMP messages. If DVMRP is used for multicast routing, you must also enable
IGMP on any interfaces used by DVMRP. To enable IGMP on an interface, enter
the command:
ENABLE IP IGMP INTERFACE=interface
IGMP keeps the local group database up to date with current multicast group
members by updating it when it hears IGMP Host Membership Reports on an
interface. If the router is the IGMP designated router for the subnetwork, it
sends out IGMP Host Membership Queries at a Query Interval. If the router
does not receive a Host Membership Report for a multicast group on an
interface within the Timeout period, it deletes the multicast group from its local
group database. The default value of the Query Interval (125 seconds) and of
the Timeout (2*(Query Interval + 10) seconds) will suit most networks. You
should only change these defaults with caution, and if you have a sound
understanding of how they affect interaction with other devices. To change the
intervals, enter the command:
SET IP IGMP [TIMEOUT=1.65535] [QUERYINTERVAL=1.65535]
To display information about IGMP and multicast group membership, enter
the command:
SHOW IP IGMP
Multicasting using DVMRP
This example (Figure 15 on page 101) allows IP hosts to send data to and
receive data from the multicast groups. Multicast group management uses
IGMP, and multicast routing between the routers uses DVMRP. The example
assumes that each router starts from the default configuration.
Multicast packets are delivered along the shortest path from one host to
another. The distance is the sum of metrics along this path. So in this example,
the shortest path from IP host A to IP host B is Router A → Router C → Router
B. From IP host A to IP host D the shortest path is Router A → Router C →
Router D. If IP host B joins the multicast group to which IP host A is a sender,
multicast data packets will not be delivered to Router D or IP host D, unless IP
host D also joins the same multicast group. Changing the metric on interfaces
may change the path by which multicast packets are delivered.
Interfaces with DVMRP enabled must also have IGMP enabled.