IP Multicast

IP Multicast enables a single host to distribute information to multiple recipients. To do this, multicast protocols use class D IP addresses to specify specific multicast groups to which information is sent. The class D IP address used by multicast routing protocols, ranges from 224.0.0.1 to 224.0.0.255. The class D IP addresses available for general use are 224.0.1.0 to 239.255.255.255.

In addition, IP multicasting distributes information to multicast groups in two specific ways:

• Multicast Forwarding - allows a switch to forward multicast traffic from the local multicast server to group members on directly attached subnetworks. If a multicast packet is forwarded to multiple interfaces on one VLAN, only one Forwarding Entry is added on the VLAN for the packet. One copy of the packet is sent to the VLAN. To configure interfaces for multicast forwarding select Internet Group Management Protocol (IGMP) for the multicast protocol.

Note: Interfaces configured for IGMP forwarding will not be able to participate in multicast sessions distributed through DVMRP Routing.

• Multicast Routing - allows a switch to deliver multicast traffic between neighboring routers and across the network using Distance Vector Multicast Routing Protocol (DVMRP).To configure interfaces for multicast routing select DVMRP for multicast protocol on all interfaces that are part of a multicast network.Since IGMP is required on all interfaces that have multicast clients, IGMP is enabled automatically on all interfaces configured for DVMRP.

Note: You must globally enable IP multicast routing in order to successfully configure IGMP or DVMRP. See "Routing Function" earlier in this chapter for instructions on enabling IP multicast routing.

This section contains the following procedures:

• Configuring IGMP
• Globally Enabling IGMP
• Modifying IGMP Interfaces
• Configuring and Modifying DVMRP
• Configuring the DVMRP Global Configuration
• Modifying a DVMRP Interface