Users Guide

1540 IPv4 and IPv6 Multicast

leads to significantly faster response times for receiving the full multicast

stream directly from the first-hop router (as opposed to the typical

bandwidth-limited stream traversing the RP).

Using PIM-DM as the Multicast Routing Protocol

Unlike PIM-SM, PIM-DM creates source-based shortest-path distribution

trees that make use of reverse-path forwarding (RPF). PIM-DM assumes that

when a sender starts sending data, all downstream routers and hosts want to

receive a multicast datagram. PIM-DM initially floods multicast traffic

throughout the network. Routers that do not have any downstream neighbors

prune back the unwanted traffic. In addition to PRUNE messages, PIM-DM

makes use of graft and assert messages. Graft messages are used whenever a

new host wants to join the group. Assert messages are used to shutoff

duplicate flows on the same multi-access network.

There are two versions of PIM-DM. Version 2 does not use the IGMP

message; instead, it uses a message that is encapsulated in an IP packet, with

protocol number 103. In Version 2, a Hello message is introduced in place of a

query message.

PIM-DM is appropriate for:

• Densely distributed receivers

• Few senders-to-many receivers (due to frequent flooding)

• High volume of multicast traffic

• Constant stream of traffic

To minimize the repeated flooding of datagrams and subsequent pruning

associated with a particular source-group (S,G) pair, PIM-DM uses a State

Refresh message. This message is sent by the router(s) directly connected to

the source and is propagated throughout the network. When received by a

router on its RPF interface, the State Refresh message causes an existing

prune state to be refreshed. State Refresh messages are generated periodically

by the router directly attached to the source.

What Is DVMRP?

DVMRP is an interior gateway protocol that is suitable for routing multicast

traffic within an autonomous system (AS). DVMRP should not be used

between different autonomous systems due to limitations with hop count and

scalability.