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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
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Chapter 15 Configuring Multicast Services
Understanding How Multicasting Works
When the CGMP/IGMP-capable router receives an IGMP control packet, it creates a CGMP or IGMP
packet that contains the request type (either join or leave), the multicast group address, and the MAC
address of the host. The router sends the packet to a well-known address to which all switches listen.
When a switch receives the packet, the supervisor engine module interprets the packet and modifies the
forwarding table automatically.
You can statically configure multicast groups using the set cam static command. Multicast groups that
are learned through CGMP or IGMP snooping are dynamic. If you specify group membership for a
multicast group address, your static setting supersedes any automatic manipulation by CGMP or IGMP.
Multicast group membership lists can consist of both user-defined and CGMP/IGMP-learned settings.
Note If a spanning tree VLAN topology changes, the CGMP/IGMP-learned multicast groups on the VLAN
are purged and the CGMP/IGMP-capable router generates new multicast group information.
If a CGMP/IGMP-learned port link is disabled for any reason, that port is removed from any multicast
group memberships. We recommend that you enable the spanning tree PortFast feature on ports to which
hosts are directly connected if you are using CGMP. For information on configuring spanning tree
PortFast, see Chapter 8, “Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast,
BackboneFast, and Loop Guard.”
Joining a Multicast Group
When a host wants to join an IP multicast group, it sends an IGMP join message specifying its MAC
address and the IP multicast group it wants to join. The CGMP/IGMP-capable router then builds a
CGMP/IGMP join message and multicasts the join message to the well-known address to which the
switches listen.
Upon receipt of the join message, each switch searches its Enhanced Address Recognition Logic
(EARL) table to determine if it contains the MAC address of the host asking to join the multicast group.
If a switch finds the MAC address of the host in its EARL table associating the MAC address with a
nontrunking port, the switch creates a multicast forwarding entry in the EARL forwarding table. The
host that is associated with that port receives multicast traffic for that multicast group. In this way, the
EARL automatically learns the MAC addresses and port numbers of the IP multicast hosts.
Leaving a Multicast Group
The CGMP/IGMP-capable router sends periodic multicast group queries. If a host wants to remain in a
multicast group, it responds to the query from the router. In this case, the router does nothing. If a host
does not want to remain in the multicast group, it does not respond to the router query. After a number
of queries, if the router receives no reports from any host in a multicast group, the router sends a
CGMP/IGMP command to the switch and requests that the switch remove the multicast group from its
forwarding tables.
Note If other hosts in the same multicast group do respond to the multicast group query, the router does not
ask the switch to remove the group from its forwarding tables. The router does not remove a multicast
group from the forwarding tables until all the hosts in the group ask to leave the group.
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