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GARP Multicast Registration Protocol
Generic Attribute Registration Protocol (GARP) Multicast Registration Protocol (GMRP) prevents multicast
flooding on the switch. This protocol provides a way for the host to communicate with the switch at the MAC
level. It runs independently of the Layer 3 protocol.
GMRP software must be running on both the host and the Cisco switch. GMRP is generally used with IGMP
on the host. The GMRP application sends a message when the host sends an IGMP message. The switch uses
the GMRP information to manage the multicast traffic. The switch periodically sends GMRP queries. If it
does not receive a response, the port will be removed from the multicast group.
GMRP requires the installation of special software on the host; for this reason, it is not widely used. This
protocol is most suitable in a situation where the switch cannot run IGMP snooping or CMGP.
Configuring IP Multicast Routing
To enable IP multicast routing, use the following command in Global mode:
Router(config)# ip multicast−routing
Note The hostname “Router” indicates that this must be configured on an external router or an internal route
processor, such as the RSM, RSFC, MSM, or MSFC. The hostname “Coriolis” indicates that the
configuration is taking place on a Set/Clear−based IOS switch, such as the 2900G, 5000, or 6000 family
of switches.
Disabling IP Multicast Routing
To disable IP multicast routing, use the following command in Global mode:
Router(config)# no ip multicast−routing
Enabling PIM on an Interface
When you enable PIM on an interface, IGMP is enabled at the same time on that interface. PIM operates in
either Dense mode or Sparse mode on each interface. The different modes dictate how the router populates the
multicast routing table, as well as how the router will forward multicast packets.
Use the following commands to configure each interface:
Router(config−if)# ip pim dense−mode
Router(config−if)# ip pim sparse−mode
You can also let the multicast group decide which mode the interface will be in:
Router(config−if)# ip pim sparse−dense−mode
Disabling PIM on an Interface
Use the following commands to disable PIM on the interface:
Router(config−if) no ip pim sparse−mode
Router(config−if) no ip pim dense−mode
Router(config−if) no ip pim sparse−dense−mode
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