Cisco WS-C6506 Software Manual page 1195

Chapter 51 Configuring Multicast Services
Periodically, the router sends out general queries to all VLANs. As the multicast receivers respond to
the router's queries, the switch intercepts them. Only the first IGMP join (report) per VLAN and per IP
multicast group is forwarded to the router. Subsequent reports for the same VLAN and group are suppressed.
The switch processor creates one entry per VLAN in the Layer 2 forwarding table for each MAC group
from which it receives an IGMP join request. All hosts that are interested in this multicast traffic send
the join requests and are added to the port list of this forwarding table entry. If a port is disabled, it is
removed from all multicast group entries.
IGMP version 3 snooping uses source-based filtering and is the industry-designated standard protocol
for the hosts to signal channel subscriptions in Source Specific Multicast (SSM). The source-based
filtering enables the hosts and routers to specify which source addresses should be allowed or blocked
for a specific multicast group. When you enable IGMP version 3 snooping on a Catalyst 6500 series
switch, the switch maintains the IGMP version 3 states based on the IGMP version 3 reports that it
receives from a port on a per-group, per-VLAN basis and either allows or blocks the source traffic on
that port based on the type of IGMP version 3 message that it receives. If the switch receives the IMGP
version 2 snooping reports for SSM, the reports are forwarded to the MSFC2 and a system error message
is generated.
For IGMP version 3 snooping, use Cisco IOS Release 12.1(11b)E1 or later releases on MSFC2.
Note
IGMP Version 3 Snooping Restrictions
The following restrictions apply to IGMP version 3 snooping:
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OL-8978-04
With software release 8.3(1), it is mandatory that you run Cisco IOS Release 12.2(17d)SXB1 if you
plan on using IGMP version 3 snooping with MMLS.
IGMP version 3 snooping should be used with PIM-SSM only. For the IGMP version 3 reports that
are received in non-SSM mode, IGMP version 2 snooping is performed.
IGMP version 3 snooping is supported for INCLUDE mode only. IGMP version 3 snooping is not
supported for EXCLUDE mode. IGMP version 3 reports pertaining to EXCLUDE mode are not
processed but are just flooded on the VLAN.
IGMP version 3 snooping will not discover the routers running Multicast OSPF (MOSPF) or
Distance Vector Multicast Routing Protocol (DVMRP) in software release 8.3(1) and later releases.
SPAN, RSPAN, private VLANs, and RGMP are not supported with IGMP version 3 snooping.
IGMP version 3 snooping is supported for Single-Router Mode (SRM) only. Although Supervisor
Engine 2 supports Dual-Router Mode (DRM), IGMP version 3 snooping does not support DRM.
IGMP version 3 snooping is not supported on the systems with a Supervisor Engine 1 or Supervisor
Engine 1A.
*, G/m hardware switching for the SSM flows is supported for the ACEs that have only the permit
action. The deny action should not be used for SSM. Configuring an ACE with the deny action when
SSM is used may cause data loss for the IGMP version 2 snooping hosts, which operate under
regular PIM sparse mode.
A system that is configured for Layer 2 switching supports only approximately 700 ACLs.
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
Understanding How Multicasting Works
51-3