Comparison of PIMDM and PIMSM
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 20-4 summarizes the important features of PIM-DM and PIM-SM.
Table 20-4 Comparison of PIM-DM and PIM-SM
KEY POINT
Table 20-4 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 & 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. |
Table 20-4 Comparison of PIM-DM and PIM-SM (Continued)
Table 20-4 Comparison of PIM-DM and PIM-SM (Continued)
|
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 |
Continue reading here: Foundation Summary Ayu
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