Igmp Snooping; Router-Port Group Management Protocol (Rgmp) - Cisco Catalyst 3650 Configuration Manual

Constraining IP Multicast in Switched Ethernet
Example: CGMP Configuration, on page 107

IGMP Snooping

IGMP snooping is an IP multicast constraining mechanism that runs on a Layer 2 LAN switch. IGMP snooping
requires the LAN switch to examine, or "snoop," some Layer 3 information (IGMP Join/Leave messages) in
the IGMP packets sent between the hosts and the router. When the switch receives the IGMP host report from
a host for a particular multicast group, the switch adds the port number of the host to the associated multicast
table entry. When the switch hears the IGMP Leave group message from a host, the switch removes the table
entry of the host.
Because IGMP control messages are sent as multicast packets, they are indistinguishable from multicast data
at Layer 2. A switch running IGMP snooping must examine every multicast data packet to determine if it
contains any pertinent IGMP control information. IGMP snooping implemented on a low-end switch with a
slow CPU could have a severe performance impact when data is sent at high rates. The solution is to implement
IGMP snooping on high-end switches with special application-specific integrated circuits (ASICs) that can
perform the IGMP checks in hardware. CGMP is a better option for low-end switches without special hardware.

Router-Port Group Management Protocol (RGMP)

CGMP and IGMP snooping are IP multicast constraining mechanisms designed to work on routed network
segments that have active receivers. They both depend on IGMP control messages that are sent between the
hosts and the routers to determine which switch ports are connected to interested receivers.
Switched Ethernet backbone network segments typically consist of several routers connected to a switch
without any hosts on that segment. Because routers do not generate IGMP host reports, CGMP and IGMP
snooping will not be able to constrain the multicast traffic, which will be flooded to every port on the VLAN.
Routers instead generate Protocol Independent Multicast (PIM) messages to Join and Prune multicast traffic
flows at a Layer 3 level.
Router-Port Group Management Protocol (RGMP) is an IP multicast constraining mechanism for router-only
network segments. RGMP must be enabled on the routers and on the Layer 2 switches. A multicast router
indicates that it is interested in receiving a data flow by sending an RGMP Join message for a particular group.
The switch then adds the appropriate port to its forwarding table for that multicast group--similar to the way
it handles a CGMP Join message. IP multicast data flows will be forwarded only to the interested router ports.
When the router no longer is interested in that data flow, it sends an RGMP Leave message and the switch
removes the forwarding entry.
If there are any routers that are not RGMP-enabled, they will continue to receive all multicast data.
Related Topics
Configuring IP Multicast in a Layer 2 Switched Ethernet Network, on page 105
RGMP Configuration Example, on page 107
OL-29890-01
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
IGMP Snooping
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