Cisco Group Management Protocol (CGMP)

Cisco Group Management Protocol (CGMP)
We have discussed IGMPv1, IGMPv2, and IGMPv3, which are open standard protocols for
host membership of multicast groups. When running multicast at layer 2, things get a little complicated
for the switch. It doesn’t know which packets are membership report messages or
which are actual multicast group data packets because all of them have the same MAC address.
Cisco Group Management Protocol (CGMP) was implemented to fill this void. It runs on both
routers and switches.
The key feature of CGMP is that it uses two MAC addresses:
Group Destination Address (GDA) The GDA is the multicast group address mapped to the
MAC multicast address.
Unicast Source Address (USA) The USA is the unicast MAC address of the host. USA enables
the host to send multicast membership reports to the multicast router�"the multicast router can
also be a Route Switch Module (RSM) or Multilayer Switch Feature Card (MSFC)�"and still tell
the switch which port needs to receive the multicast data.
In addition to being able to make port assignments on the switch, CGMP also handles the
interface assignment on the router. If a switch doesn’t have any ports that need to receive multicast
data, CGMP informs the router that it doesn’t need to forward multicast group data out
the router interface.

CGMP uses many of the same processes IGMP uses. The main difference is that
CGMP is used between the router and switch. When switches are involved, the IGMP
requests must be translated to CGMP and passed on to the switch. These processes include
the following:
 CGMP Join process
 Switch host management
 CGMP Leave process