Cisco Catalyst 4224 Release Notes page 9

IGMP Snooping Support
One or more users might need to connect to local/remote server(s) via IP WAN or other LAN connection
to access a MULTICAST application, such as IPTV or WEBCast. The Catalyst 4224 should send the
multicast stream only to the requesting user PC and should not flood the rest of user PCs with such
streams.
IGMP is a multicast protocol that directly affects PCs (hosts). IGMP allows hosts to inform Layer 3
devices (routers) that they want to receive multicast traffic for a specific multicast group address.
Multicast Address Range
The multicast address range includes any IP address in the range 224.0.0.0 to 239.255.255.255. The
range of addresses between 224.0.0.0 and 224.0.0.255 (inclusive) is RESERVED for the use of routing
protocols and other low-level topology discovery or maintenance protocols.
Any multicast server defined in the network will be part of one or more multicast groups.
Whenever a user's PC (referred to as host) requests the services of a multicast server, for example, it asks
for an application (such as a movie channel), and it will send a message to join the particular group
(join-group) the server uses to multicast (transmit) such movie.
The Catalyst 4224 will send the movie stream only to the hosts that have sent a join request. The
Catalyst 4224 keeps track of the multicast entries via two tables known as Multicast address count and
IGMP snooping count. As in any system, these tables are finite, and specifically in the Catalyst 4224
their max value is 255. Once this value is reached, further joins (join-group) or VLAN definitions cannot
be executed.
Other IGMP Requirements
In order to enable IGMP within the Catalyst 4224, a multicast protocol such as PIM must be configured
for the corresponding interface(s), which are mainly VLAN interfaces. The user cannot enable IGMP
directly as it is done in other Cisco switches.
TIPS on how to keep track of the multicast tables to prevent them from getting full (based on test
observations),
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OL-1798-01
For every VLAN added to the system, the multicast address count will increase by 4. (IGMP
snooping address Count does not get impacted by VLAN addition.)
Whenever a multicast protocol is added (for example, ip pim dense-mode) both counters increase
by 3. (The first instance will increase counters by 4 because Multicast group 224.0.1.40 is added
too.)
You can add up to 62 VLANs without IGMP support. Any additional VLAN will get added, but cdp
and EIGRP for these VLANs will not work (among other things).
When a multicast group is added via the join-group command, both counters are increased by 1.
When you delete a VLAN interface that was multicasting (for example, pim) no counters get
decreased. (A bug report exists for this issue.)
When you delete a multicast protocol (for example, no ip pim value) from an interface, the counters
get increased by 1.
When a multicast group leaves (via the no ip igmp join-group ip-address), both counters get properly
decremented by 1. This is the case when a host finishes using the multicast application, like viewing
a movie.
Important Notes
IGMP Snooping Support
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