Comparison of PIM-DM and PIM-SM
One of the most confusing parts of the PIM-DM and PIM-SM designs is that it appears that if
sources keep sending, and receivers keep listening, there is no difference between the end results
of the end-user multicast packet flow using these two options. Once PIM-SM completes its more
complicated processes, the routers near the receivers have all joined the SPT to the source, and the
most efficient forwarding paths are used for each (S,G) tree.
Although its underlying operation is a bit more complicated, PIM-SM tends to be the more
popular option today. PIM-SM’s inherent strategy of not forwarding multicasts until hosts request
them makes it more efficient during times of low usage. When the numbers of senders and
receivers increases, PIM-SM quickly moves to use the SPT—the same SPT that would have been
derived using PIM-DM. As such, PIM-SM has become a more popular option for most enterprise
implementations today. It has also become a popular option for interdomain multicast as well.
Table 17-4 summarizes the important features of PIM-DM and PIM-SM.
Table 17-4 Comparison of PIM-DM and PIM-SM
Feature PIM-DM PIM-SM
Destination address for
Version 1 Query messages,
and IP protocol number
224.0.0.2 and 2 224.0.0.2 and 2
Destination address for
Version 2 Hello messages,
and IP protocol number
224.0.0.13 and 103 224.0.0.13 and 103
Default interval for Query
and Hello messages
30 seconds 30 seconds
Default Holdtime for
Versions 1 and 2
90 seconds 90 seconds
Rule for electing a
designated router on a
multiaccess network
Router with the highest IP
address on the subnet
Router with the highest IP address
on the subnet
Main design principle A router automatically receives
the traffic. If it does not want the
traffic, it has to say no (send a
Prune message) to its sender.
Unless a router specifically makes a
request to an RP, it does not receive
multicast traffic.
Feature PIM-DM PIM-SM
SPT or RPT? Uses only SPT First uses RPT and then switches to
SPT
Uses Join/Prune messages? Yes Yes
Uses Graft and Graft-Ack
messages?
Yes No
Uses Prune Override
mechanism?
Yes Yes
Uses Assert message? Yes Yes
Uses RP? No Yes
Uses source registration
process?
No Yes