Cisco Systems 78-11741-02 manual Enabling PIM Nonbroadcast Multiaccess Mode, Enables PIM Nbma mode

Configuring IP Multicast Routing

Advanced PIM Features Configuration Task List

For traffic from DVMRP neighbors, proxy registering is always active and cannot be influenced by the ip pim dense-modeproxy-registerinterface configuration command. For dense mode or DVMRP regions, proxy registering allows for limited interoperability between a dense mode region and a sparse mode domain. This limitation is referred to as “receiver must also be sender.” The “receiver must also be sender” limit exists because there is no mechanism in dense mode protocols to convey the existence of receiver-only hosts to a border router, and the flooding (and pruning) of all multicast traffic originated in the dense mode domain inhibits the purpose of a sparse mode domain. The behavior of participating hosts in the dense mode region is as follows:

•A host in the dense mode region is only guaranteed to receive traffic from sources in the sparse mode domain through the proxy registering border router if at least one host is in the dense mode region that is a sender for the multicast group. This host is typically the receiving host itself.

•A sender in the dense mode region will trigger proxy registering in the border router, which in turn will cause the border router to join the multicast group and forward traffic from sources in the sparse mode domain toward the dense mode region.

•If no sender is in the dense mode region for a multicast group, then no traffic will be forwarded into the dense mode region.

Enabling PIM Nonbroadcast Multiaccess Mode

PIM nonbroadcast multiaccess (NBMA) mode allows the Cisco IOS software to replicate packets for each neighbor on the NBMA network. Traditionally, the software replicates multicast and broadcast packets to all “broadcast” configured neighbors. This action might be inefficient when not all neighbors want packets for certain multicast groups. NBMA mode enables you to reduce bandwidth on links leading into the NBMA network, and to reduce the number of CPU cycles in switches and attached neighbors.

Configure this feature on ATM, Frame Relay, Switched Multimegabit Data Service (SMDS), PRI ISDN, or X.25 networks only, especially when these media do not have native multicast available. Do not use this feature on multicast-capable LANs (such as Ethernet or FDDI).

You should use PIM sparse mode with this feature. Therefore, when each join message is received from NBMA neighbors, PIM stores each neighbor IP address and interface in the outgoing interface list for the group. When a packet is destined for the group, the software replicates the packet and unicasts (data-link unicasts) it to each neighbor that has joined the group.

To enable PIM NBMA mode on your serial link, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip pim nbma-mode	Enables PIM NBMA mode.

Consider the following two factors before enabling PIM NBMA mode:

•If the number of neighbors grows, the outgoing interface list gets large, which costs memory and replication time.

•If the network (Frame Relay, SMDS, or ATM) supports multicast natively, you should use it so that replication is performed at optimal points in the network.

Cisco IOS IP Configuration Guide

IPC-428