Allied Telesis AR700 SERIES Software Release 2.7.1 - Manual (Page 91)

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Software Release 2.7.1
C613-02047-00 REV A
switches on a single physical network to establish hosts’ membership in
particular multicast groups.
The router uses this information, in conjunction with a multicast routing
protocol, to know which other routers to route multicast traffic to. The router
maintains a routing table for multicast traffic with Distance Vector Multicast
Routing Protocol (DVMRP), Protocol Independent Multicast-Sparse Mode
(PIM-SM), or Protocol Independent Multicast-Dense Mode (PIM-DM). You
must configure IGMP and one of the multicast routing protocols before the
router can forward multicast packets. DVMRP and PIM-Sparse Mode share a
separate multicast forwarding table.
When the router receives a packet addressed to a multicast group, it forwards it
to the interfaces that have group members connected to them, according to
IGMP, and out other interfaces specified by the multicast routing protocol.
Membership in a multicast group is dynamic; hosts can join and leave at any
time. Multicast groups can be long or short lived, and can have relatively stable
or constantly changing membership. There is no limit on the location or
number of members in a multicast group. A host can belong to more than one
multicast group at a time.
When the router finds out from IGMP that a new host has joined a multicast
group on one of its interfaces, the router needs to receive the multicast traffic
for this group, so that it can forward it to the host. The router uses the multicast
routing protocol (DVMRP, PIM-SM or PIM-DM) to notify routers closer to the
sender (upstream) to forward it traffic for the group.
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