Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring Multicast Layer 2 (Rev. 03)
IGMP Snooping
© 2008 Foundry Networks, Inc.
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Leaving a Multicast Group
In IGMP version 1, if a host does not want to receive the IGMP traffic, it just silently quits the
group. IGMP multicast routers periodically send host membership query messages to discover if
any member is still interested in the specific multicast group traffic. As long as the IGMP
Snooping switch receives this Query Group message, it forwards the message to the associated port
included in the multicast group. If the router does not receive a Report Group message after three
consecutive queries, it deletes the GDA MAC of the associated port in the MAC Filtering Database.
In IGMP version 2, if a host does not want to receive the IGMP traffic any more, it sends a Leave
Group message. When the IGMP Snooping switch receives this Leave Group message, it sends an
IGMP group specified query message to determine if any device behind that port is interested in
the specific multicast group traffic. If the device does not receive any IGMP Report message, it
removes the GDA MAC address from the associated port in the MAC Filtering Database.
Immediate Leave
IGMP Snooping Immediate Leave processing allows the device to remove an interface that sends a
Leave message from the forwarding table without first sending out group-specific queries to the
interface. The port is pruned from the multicast tree for the multicast group specified in the original
Leave message. Immediate Leave processing ensures optimal bandwidth management for all hosts
on a switched network, even when multiple multicast groups are in use simultaneously.
NOTE
IGMP Snooping Immediate Leave is suitable only if one receiver is connected on
the port.
Aging a Multicast Group
When a report is received (unsolicited or in response to a query), the IGMP snooping sets the age
timer to this entry. If the report is received and the multicast group already exists, the IGMP
snooping just updates the age timer.
Once the age timer expires, the report is removed from the IGMP snooping table and the entry in
Multicast Forwarding Table is updated.
The calculation of the age timer of a report is as follow:
Report Age = robustness * query-in response-time
The default value is 260 seconds.
Multicast Routers and Multicast Servers
A
Multicast
router
(
mrouter
) is a router that runs a multicast routing protocol (such as PIM) and
participates in the multicast tree. On the edge of the network, a multicast router might be connected
to an IGMP Snooping switch. The port on which the multicast router is connected is called an
mrouter port
. The Multicast router sends periodic General IGMP queries.
The snooping device identifies an mrouter port by either receiving an IGMP query on that port, or
by explicit configuration of the port as an mrouter port (by using the
ip igmp snooping vlan
mrouter
command in Global Configuration mode).
A multicast
server
may be any stream server sending multicast traffic (such as a UDP stream
destined to multicast address). As a rule, a multicast server does not send IGMP queries. The
snooping switches connected to a multicast server (which does not send queries) require additional