Understanding How Gmrp Works; Understanding How Rgmp Works - Cisco WS-C6506 Software Manual

Understanding How Multicasting Works

Understanding How GMRP Works

GMRP is a Generic Attribute Registration Protocol (GARP) application that provides a constrained
multicast flooding facility similar to IGMP snooping. GMRP and GARP are industry-standard protocols
that are defined by the IEEE. For detailed protocol operational information, refer to 802.1p.
The GMRP software components run on both the switch and on the host. (Cisco is not a source for
GMRP host software.) On the host, in an IP multicast environment, you must use IGMP with GMRP; the
host GMRP software spawns the Layer 2 GMRP versions of the host's Layer 3 IGMP control packets.
The switch receives both the Layer 2 GMRP and the Layer 3 IGMP traffic from the host. The switch
forwards the Layer 3 IGMP control packets to the router and uses the received GMRP traffic to constrain
the multicasts at Layer 2 in the host's VLAN.
When a host wants to join an IP multicast group, it sends an IGMP join, which spawns a GMRP join.
When the switch receives the GMRP join, it adds the port through which the join was received to the
appropriate multicast group. The switch propagates the GMRP join to all the other hosts in the VLAN,
one of which is typically the multicast source. When the source is multicasting to the group, the switch
forwards the multicast only to the ports from which it received the join messages for the group.
The switch sends periodic GMRP queries. If a host wants to remain in a multicast group, it responds to
the query and the switch does nothing. If a host does not want to remain in the multicast group, it can
either send a leave message or not respond to the periodic queries from the switch. If the switch receives
a leave message or receives no response from the host for the duration of the leaveall timer, the switch
removes the host from the multicast group.
To use GMRP in a routed environment, enable the GMRP forwardall option on all ports where the
Note
routers are attached. (See the
page

Understanding How RGMP Works

Without RGMP, all multicast routers receive all multicast data traffic entering the switch. With RGMP, a
multicast router can request not to receive the multicast traffic if that router has no downstream receivers for
the multicast traffic. Catalyst 6500 series switches support RGMP, which enables a switch to reduce
network congestion by forwarding the multicast data traffic only to those routers that are configured to
receive it.
Note To use RGMP, you must enable IGMP snooping on the switch and Protocol Independent Multicast
(PIM) on the routers. Only PIM sparse mode is currently supported.
All routers on the network must be RGMP capable. The RGMP-capable routers periodically send an
RGMP hello message to the switch. The RGMP hello message tells the switch not to send the multicast
data to the router unless an RGMP join has also been sent to the switch from that router. When an RGMP
join is sent, the router is able to receive the multicast data. To learn how to set a router to receive the
RGMP data, see the
To stop receiving the multicast data, a router must send an RGMP leave message to the switch. To
disable RGMP on a router, the router must send an RGMP bye message to the switch.
Table 51-1
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
51-6
"Enabling the GMRP Forward-All Option on a Switch Port" section on
51-22.)
"RGMP-Related CLI Commands" section on page
provides a summary of the RGMP message types.
Chapter 51 Configuring Multicast Services
51-34.
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