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Cisco Switching Black Book - Sean Odom, Hanson Nottingham.pdf
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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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