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PIM Sparse-Dense-Mode

I can't seem to grasp the sheer concept of pim sparse dense mode.   Documentation says that an enabled interface in pim sparse dense mode will use dense mode for dense groups and sparse mode for sparse group.  It says that ip pim sparse dense mode can be used in situations such as Auto-RP where candidate RPs and mapping agents need to communicate with multicast traffic; however the RP isnt known which results in the catch 22.  I understand the the two groups used by auto-rp in sparse dense mode will use dense mode for the 224.0.1.39, 40 and sparse mode for client traffic.   That part I understand and understand the reasoning for ip pim sparse dense mode.

My question is why is an RP always needed in sparse-dense-mode whether through Auto-RP or statically.  It seems like regular client multicast traffic would fall back to dense mode if there wasnt an RP enabled.  Although, I know its not considered as efficient, multicast traffic should still be forwarded through normal dense mode operation.

7 REPLIES
VIP Super Bronze

Re: PIM Sparse-Dense-Mode

My question is why is an RP always needed in sparse-dense-mode whether through Auto-RP or statically.  It seems like regular client multicast traffic would fall back to dense mode if there wasnt an RP enabled.  Although, I know its not considered as efficient, multicast traffic should still be forwarded through normal dense mode operation.


Hi,

Sparse-dense mode, as the name implies, allows the interface to operate on a per-group basis in either sparse or dense mode. A group specified as dense is not mapped to an RP. Instead, data packets destined for that group are forwarded by means of PIM dense-mode rules. A group specified as sparse is mapped to an RP, and data packets are forwarded by means of PIM sparse-mode rules.

HTH

Reza

New Member

Re: PIM Sparse-Dense-Mode

Thats what the documentation says as well.  What determines that the group is specified as dense or another group is specified as sparse?  The cisco router/switch says it can operate either way, so something else has to specify how the group operates.

VIP Super Bronze

Re: PIM Sparse-Dense-Mode

some environments might require PIM to run in a single region in sparse mode for some groups and in dense mode for other groups.

Have a look at the config guide:

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfmulti.html#wp1001103

New Member

Re: PIM Sparse-Dense-Mode

Yep, and thats the info that I have read before.  I guess the issue Im running into is environments

where ip pim sparse dense mode is configured; however, an rp is not configured.  multicast routing doesnt seem to work until I configure an rp address.  However, this shouldnt happen.  Multicast routing should fall back to dense mode and multicast routing should work fine.  It also always seems that TAC wants an RP always configured as well.

Hall of Fame Super Blue

Re: PIM Sparse-Dense-Mode

tsmarcyes wrote:

Yep, and thats the info that I have read before.  I guess the issue Im running into is environments

where ip pim sparse dense mode is configured; however, an rp is not configured.  multicast routing doesnt seem to work until I configure an rp address.  However, this shouldnt happen.  Multicast routing should fall back to dense mode and multicast routing should work fine.  It also always seems that TAC wants an RP always configured as well.

There is a difference between -

1) having no RP so there is no RP to query

2) having an RP and querying it and then finding that it does not have a mapping for the group

pim sparse-dense mode will fall back to dense mode but fallback only happens after the RP has been queried. Without an RP it doesn't work.

Jon

New Member

Re: PIM Sparse-Dense-Mode

jon.marshall wrote:

tsmarcyes wrote:

Yep, and thats the info that I have read before.  I guess the issue Im running into is environments

where ip pim sparse dense mode is configured; however, an rp is not configured.  multicast routing doesnt seem to work until I configure an rp address.  However, this shouldnt happen.  Multicast routing should fall back to dense mode and multicast routing should work fine.  It also always seems that TAC wants an RP always configured as well.

There is a difference between -

1) having no RP so there is no RP to query

2) having an RP and querying it and then finding that it does not have a mapping for the group

pim sparse-dense mode will fall back to dense mode but fallback only happens after the RP has been queried. Without an RP it doesn't work.

Jon

How does it work with Auto-RP then.  The mapping agents and RPcandidates send their "advertisements" as multicast. According to the documentation, the multicast groups will fall back to dense mode to pass the "multicast advertisements" to each other .  However, there isnt an "orignal RP" to query first and then to fall back to dense mode like you were saying in your post.  What am I missing here?

The protocol should be really simple.  If there is an RP for the multicast group,then use sparse mode,if not use dense mode  However, this doesnt seem to be the case an an RP seems to always be needed in one way or another even just to query in the beginning to later fall back to dense mode.

Hall of Fame Super Silver

Re: PIM Sparse-Dense-Mode

Hello Tsmarcyes,

if sh ip pim rp mapping

does not provide a candidate RP for the group, then the group is considered and treated as dense mode.

This requires that all multicast routers have to agree on mapping results or it will be a problem.

Depending on network size autoRP or bootstrap protocol can be used to propagate candidate RP information including mapping and to keep all devices informed and on sync

Hope to help

Giuseppe

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