Multicast routing

[edison65]

I have a single layer 3 switch with SVI vlan 10 and 50.

VLAN 10 is for data and vlan 50 is where my streaming server lives.

Do i need to configure PIM sparse mode and RP ?

[M02@rt37]

Hello @edison65 

You need to route multicast ? Or activate igmp on a LAN side ?

[Joshqun Ismayilov]

Hi @edison65 

It depends on whether your streaming traffic is unicast or multicast and whether it needs to cross VLANs.

Thanks !

[paul driver]

Hello
for ASM MC and providing the mc source and receivers are in each subnet then yes you will require it .

[Joseph W. Doherty]

"Do i need to configure PIM sparse mode and RP ?"

No, because DM would be an option and it doesn't need/use a RP.

[paul driver]

Hello @Joseph W. Doherty  
For the sake of a single static RP i would prefer SM to negate the flooding or you could meet me in the middle and use S/DM?

[Joseph W. Doherty]

". . . would prefer SM to negate the flooding or you could meet me in the middle and use S/DM?"

Flooding, what flooding?

Actually, flooding is the major distinction between DM and SM, but let's suppose (which I assumed to be the case - not withstanding how assume may be parsed - laugh) the single L3 switch supports and has enabled IGMP snooping.  If no receiving clients want any multicast, what's flooded?

If the case multiple multicast routers were involved, or IGMP snooping not being used (especially the latter), using SM usage would be worth considering.

I'm not against SM or DM/SM, combination, but if OP wants to avoid SM, for described topology, don't see a compelling reason to use it.  (BTW, I've supported DVMRP, non-Cisco, and DM, Cisco, in a good size mixed hardware environment.  Ugly, yes, due to interoperability issues of DVMPR and DM, but flooding, per se, wasn't impactful, even though this was principally to support Americas wide company video presentations.)

[M02@rt37]

Agree with @paul driver. Using a single static RP with PIM SM is a reasonable trade-off to eliminate flooding while keeping the configuration simple. SM ensures that multicast traffic is only forwarded when explicitly requested, avoiding the periodic reflooding behavior of DM. In PIM DM, the flood-and-prune process is periodic. After 3 minutes, the prune state expires, and the multicast traffic is flooded again. This is because DM does not maintain long-term multicast group state efficiently; instead, it periodically refloods the traffic to ensure no receivers have joined since the last pruning. In contrast, PIM SM does not flood traffic periodically. It forwards multicast only when explicitly requested via PIM Join msg, making it more scalable for networks with fewer REC...

[Joseph W. Doherty]

Yep, all true for SM, but again, assuming same L3 switch multicast client network is using IGMP snooping, where's the flooding packets going?

Effectively, unless there's at least one client that wants the multicast stream, the SVI should black hole any undesired flooded packets.

I.e. L3 switch effectively delivers multicast packets, or not, in this particular case, alike.

If you want to argue that SM is "better", in more complex situations, including not having IGMP snooping, I completely agree.

[M02@rt37]

You're correct—if IGMP snooping is enabled and working properly, then in a single L3 switch scenario, DM flooding should be minimal or nonexistent. The switch will only forward multicast traffic to interfaces where receivers have explicitly requested it via IGMP joins...

If there are no interested clients, the traffic should be dropped at the switch's SVI, effectively preventing unnecessary flooding.

The main concern with DM in more complex topologies arises when multiple multicast routers are involved, as periodic flooding restarts every three minutes unless pruned. In a single L3 switch deployment with IGMP snooping, that issue doesn't really apply. SM does offer better efficiency in larger networks, particularly where you have multiple routers, sparse receivers, or WAN links, but in this simple case, there's little functional difference between DM and SM.

If the goal is to keep things simple and the network topology is limited to a single L3 switch, DM can work fine without excessive flooding. However, if there's any potential for network expansion or multiple multicast routers, SM would still be the better long-term choice.