Information Technology Reference
In-Depth Information
Without CGMP
With CGMP
A
B
C
D
A
B
C
D
Figure 11-25
CGMP
When a router receives an IGMP report, it processes the report and then sends a CGMP
message to the switch. The switch can then for ward the multicast messages to the port with
the host receiving multicast traffic. CGMP fast-leave processing allows the switch to detect
IGMP Ve r s i o n 2 leave messages sent by hosts on any of the switch ports. When a host
sends the IGMPv2 leave message, the switch can then disable multicasting for the port.
CGMP is no longer used and is not a CCDA topic. IGMP snooping is the standards-based
protocol used in today's networks.
IGMP Snooping
IGMP snooping is standards-based method for switches to control multicast traffic at
Layer 2. It has replaced CGMP. It listens to IGMP messages between the hosts and routers.
If a host sends an IGMP query message to the router, the switch adds the host to the mul-
ticast group and permits that port to receive multicast traffic. The port is removed from
multicast traffic if the host sends an IGMP leave message to the router. The disadvantage
of IGMP snooping is that it has to process every IGMP control message, which can im-
pact the CPU utilization of the switch.
Sparse Versus Dense Multicast
IP multicast traffic for a particular (source, destination group) multicast pair is transmitted
from the source to the receivers using a spanning tree from the source that connects all the
hosts in the group. Multicast destinations are represented in the following form: (S,G) or
(*,G). Any multicast transmission has a Class D multicast group address, G. A multicast
group can have more than one source, and each such source will also have a “regular”
(Class A, B or C, or CIDR) Internet address, S. The notation (*,G) means every possible
source for a given group G, while (S,G) means a particular source, at a particular Internet
address S, in the group G.
