Basic question on multicast (sparse-mode vs. sparse-dense-mode)

Answered Question
Jul 5th, 2009
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Imagine one is not 100% sure regarding number of hosts and behavior and growth of multicast applications in the network.


Some folks on my team believe that configuring 'sparse-mode' is appropriate. If I am unsure, is there any issue or problem if I configure on my router interfaces 'ip pim sparse-dense-mode'? I mean, why not deploy always ip pim sparse-dense-mode instead of sparse-mode since the former would cover both dense and sparse?

Correct Answer by thotsaphon about 7 years 8 months ago

Marlon,

It depends on bandwidth usage on your network. As you know,dense mode always flood/prune multicast packets on your network. Sparse mode will send multicast packets when someone asks for. In case you have a few receiver on your network it would be sparse mode for that. As per your question "ip pim sparse-dense-mode" is a good command as you mentioned. It will use dense mode for group that doesn't know about the RP. For sparse mode 239.0.1.39 and 239.0.1.40 are still flooded as dense. So if you use an "ip pim sparse-dense-mode" command it would be fine. You can also use "ip pim sparse-mode" and "ip pim autorp-listener" to get rid of that problem.


HTH,

Toshi

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Correct Answer
thotsaphon Sun, 07/05/2009 - 10:52
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Marlon,

It depends on bandwidth usage on your network. As you know,dense mode always flood/prune multicast packets on your network. Sparse mode will send multicast packets when someone asks for. In case you have a few receiver on your network it would be sparse mode for that. As per your question "ip pim sparse-dense-mode" is a good command as you mentioned. It will use dense mode for group that doesn't know about the RP. For sparse mode 239.0.1.39 and 239.0.1.40 are still flooded as dense. So if you use an "ip pim sparse-dense-mode" command it would be fine. You can also use "ip pim sparse-mode" and "ip pim autorp-listener" to get rid of that problem.


HTH,

Toshi

Istvan_Rabai Sun, 07/05/2009 - 11:49
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Hi Marlon,


Sparse-dense mode was introduced by Cisco to support AutoRP:

Initially, when no routers know about the RP(s), AutoRP RP-announce and RP-discovery messages (multicast groups 224.0.1.39 and 224.0.1.40 respectively) are sent using dense mode across the network.


There is a standardized solution to that:

bootstrap protocol.


The following commands configure it:

ip pim rp-candidate

ip pim bsr-candidate


If you use the bootstrap protocol, you only need to use sparse-dense mode.

In addition, as the bootstrap protocol is a standard, your solution will be vendor-independent.


Cheers:

Istvan

Giuseppe Larosa Sun, 07/05/2009 - 12:20
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Hello Istvan /Marlon,


I agree with Istvan: sparse-dense-mode has been introduced to make auto-RP to work.


only point clearly a mistyping is:


>>If you use the bootstrap protocol, you only need to use sparse-dense mode.

In addition, as the bootstrap protocol is a standard, your solution will be vendor-independent.


if you use bootstrap protocol you can use sparse-mode only everywhere.


PIM SM provides more control on BW resources used on the WAN links: for the different paradigm : it requires explicit joins and not explicit prunes.


Besides this, Cisco has introduced an alternate way to support auto-RP:


interface command

ip pim autorp listener


see


http://www.cisco.com/en/US/docs/ios/ipmulti/command/reference/imc_04.html#wp1012925


The issue with ip pim sparse-dense-mode is the fallback to dense mode when no RP mapping exists for a group G.


This can be mitigated with:

global command

no ip pim dm-fallback


http://www.cisco.com/en/US/docs/ios/ipmulti/command/reference/imc_04.html#wp1013545


So there are many options but bootstrap protocol has many advantages over auto-RP:

besides being multi-vendor it provides the capability to have redundancy also of the boostrap router that is elected dynamically, auto-RP mapping agent is statically configured.

the hash function provides a way to use in load balancing two candidate RPs for a multicast address range.


Hope to help

Giuseppe




Istvan_Rabai Sun, 07/05/2009 - 21:02
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Hi Giuseppe,


Thank you for your correction.

I really was thinking forward about something different and mistyped sparse-dense mode.


So the correct statement from me is:

If you use the bootstrap protocol, you only need to use SPARSE-MODE.


Cheers:

Istvan

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