Why doesn't NX-OS support PIM dense mode?

[younsooda]

Why doesn't NX-OS support PIM dense mode?

Is there any technical problem?

Cisco NX-OS PIM

Cisco NX-OS supports multicasting with Protocol Independent Multicast (PIM) sparse mode. PIM is IP routing protocol independent and can leverage whichever unicast routing protocols are used to populate the unicast routing table. In PIM sparse mode, multicast traffic is sent only to locations of the network that specifically request it. PIM dense mode is not supported by Cisco NX-OS.

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/nx-os/multicast/configuration/guide/b_multicast/b_multicast_chapter_0101.pdf

[Peter Paluch]

Hello,

I do not believe that there is any technical problem with supporting Dense Mode. However, the way I see it, it has been an universal agreement for quite some time that PIM-Dense Mode is an inferior and deprecated mode of operating multicast, it is not even envisioned for IPv6 at all, and especially in data centers for which Nexus switches are designed, flooding multicast using dense mode rules could be harmful to the overall network operation. That is what I personally see as the reasoning behind the lack of PIM-DM support in NX-OS.

Best regards,
Peter

[JAN MARIS]

Hi Peter,

I heard this appreciation statement from Cisco about PIM-DM as well many times. To a large extent I can agree. 

However, there are some scenarios for which PIM-DM simply is the best protocol, e.g.

• low-bandwidth application,
• a [relatively] high number of sources, in a [relatively] high number of source sites,
• sources can multicast data to a few different groups,
• relatively little number of receiver sites, with multiple end-points listening to the same flows, 
• receivers do not see the benefit of using IGMP because of "always-on" / static operation and all groups need to be received permanently.

As an example, think of this as telemetry type of scenario (maybe in a context of IoT).

Add to this a serious high-availability requirement (duplication) and I'd think that PIM-DM is perhaps not that bad after all: simple configuration, simple operation, quite reliable and no need for IGMP with static IGMP configuration either, no need for RP design.

The notorious flooding is not a problem because it's low bandwidth and we have PIM Dense Mode State Refresh as well. In a WAN environment, GRE tunnels are probably needed, but that is not a problem either.

Would there be a better alternative using PIM-SM or PIM-SSM?

Best regards,
Jan