DELL N3000 User Manual

Managing IPv4 and IPv6 Multicast

sending the encapsulated Register messages. This removes the load from 

the CPU of the first-hop router and the RP, as they no longer need to 

encapsulate and de-encapsulate register messages with multicast data.

These optimizations significantly reduce the load on first-hop routers and 

RPs to encapsulate/de-encapsulate PIM register messages and their associated 

multicast data. In addition, the switchover to the SPT is initiated 

immediately upon the first multicast packet reaching the last-hop router. This 

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). 

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 package, 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