working PIM-Sparse Dense mode need to add one Sparse mode to same group.

[TTFreer]

I have a singlewire informacast setup working with PIM Sparse Dense mode across multaple sites. I have one meraki enviroment that I need to set up multicast routing to as well.

currently all my interfaces are configured with "ip pim sparse-dense-mode" and nothing else. My meraki equipment only suppors PIM-SM. I know for SM I need to set a RP and am wondering if I set a RP will that break my other sites on Sparse-Dense mode and what is the best way to set that up.

topology working down from source to destination. 

informacast Multicast IP

239.0.1.2

ISR 4451

ip multicast-routing distributed

interface GigabitEthernet0/0/0
ip address x.x.101.1 255.255.255.0
ip pim sparse-dense-mode
ip igmp query-interval 125

Cat 4500-x

ip multicast-routing

interface Vlan61 (Vlan to WAN)
ip address x.x.254.1 255.255.255.0
ip pim sparse-dense-mode
!
interface Vlan101 (same subnet as ISR 4451)
ip address x.x.101.3 255.255.255.0
ip pim sparse-dense-mode

remote sites configured the same, with different IP's/Subnets 

ip multicast-routing

interface Vlan61 (Vlan of Wan)
ip address x.x.254.18 255.255.255.0
ip pim sparse-dense-mode
!
interface Vlan101(reciving devices vlan)
ip address x.x.101.1 255.255.255.0
ip pim sparse-dense-mode

[MarcSims]

As I understand it, Sparse-Dense mode will operate in Sparse mode if there is a RP for the group - so if you create a RP in your Meraki environment, this (as per normal PIM-SM config) should be advertised to all multicast enabled routers - so this group will then become Sparse-Mode for the entire multicast domain.

The question I have is why would you want to operate in Dense-mode rather than Sparse-mode? If there is no application / service reason, then I would test creating a PIM RP (at least 2 RP's - for resilience - with 2 BSR candidates to propogate RP information) & operating the service in Sparse-mode. If you need to continue to operate in Dense-Mode, then you will need to have a multicast boundary between the two environments to avoid it switching a network wide switch to Sparse-mode operation just for the group (assuming you scope the PIM RP accordingly).