Multicast - ip cgmp

[opers13]

I came across Multicast config on a 3750G and it has both:

ip pim sparse-dense-mode
ip cgmp

enabled under the SVI.

When I do "sh ip cgmp" nothing comes up and actually it's not even supported....I read cgmp was developed for CATOS??

thanks

[Giuseppe Larosa]

Hello Opers13,

CGMP is a Cisco proprietary protocol used between Cisco routers and LAN switches.

Its peculiar characteristics is that the router speaks sending frames on the CGMP multicast address and the switches listen to these messages.

The messages perform a mapping of L3 IGMP reports into L2 information (multicast group address, recevier MAC address).

It had been introduced for avoiding high cpu because old switches had not a specific IGMP snooping ASIC chip.

Now, IGMP snooping is used on the LAN switches that have an ASIC to inspect IGMP packets efficiently.

if all of your switches are new and support IGMP snooping you can remove the command

Hope to help

Giuseppe

[Ganesh Hariharan]

I came across Multicast config on a 3750G and it has both:
 ip pim sparse-dense-mode
 ip cgmp
enabled under the SVI.
When I do "sh ip cgmp" nothing comes up and actually it's not even supported....I read cgmp was developed for CATOS??
thanks

Hi,

ip pim sparse-dense-mode command treats the interface in either sparse mode or dense mode of operation, depending on which mode the multicast group operates in. The mode describes how the Cisco IOS software populates its multicast routing table and how the software forwards multicast packets it receives from its directly connected LANs. Dense mode interfaces are always added to the table when the multicast routing table is populated. Sparse mode interfaces are added to the table only when periodic join messages are received from downstream routers, or there is a directly connected member on the interface.

ip cgmp when enabled on an interface, this command triggers a CGMP join message. This command should be used only on 802 media (that is, Ethernet, FDDI, or Token Ring) or ATM.When a no ip cgmp command is issued, a triggered CGMP leave message is sent for the MAC address on the interface for group 0000.0000.0000 (all groups). CGMP can run on an interface only if Protocol Independent Multicast (PIM) is configured on the same interface. Issuing the ip cgmp command on a router enables that router to send CGMP self-join and CGMP self-leave messages as well as other types of CGMP messages.

ip cgmp is supported in cisco ios stream also.

Check out the below link for more information

http://www.cisco.com/en/US/docs/ios/12_3/ipmulti/command/reference/ip3_i1g.html#wp1100120

Hope to Help !!

Ganesh.H

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