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I am currently testing how to route Multicast on a single L3 switch. I am currently testing 2 Vlans. I researched and I know I have to use PIM or Mrouter port for this scenario but that is for when you have a dedicated router. I can't wrap it on my head on how to do it on a L3 switch only. Any advice?

Two vlan interfaces where do I assign the mrouter port so they can pass multicast on each other?

Many Thanks, 6

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Hi an welcome to network engineering.

The mrouter port is a concept from multicasting at L2 (in extenso: IGMP snooping). It does not pertain to multicast routing in the strict sense.

If you have a L3 switch that supports multicast routing (PIM), there is no need for an explicit mrouter port. The SVI ("interface vlan"), as soon as enabled for multicast routing, will process IGMP Join messages by end systems, and will also send out IGMP queries.

That being said, it is still a good idea to turn on IGMP snooping for the given VLANs, to ensure proper forwarding of multicast at the swichting layer - at the same time, IGMP snooping will detect the "internal switchport towards the SVI" as the mrouter port.

  • enable multicast routing on the switch
  • enable the relevant SVIs for PIM [1] along with the bits the selected PIM mode requires. [2]

[ADD-ON1 - Answer to comments]

in my case I don't need the rendezvous point address? cause I only have 1 layer 3 switch right?

If you choose PIM sparse mode, then (at least by principle) you need an RP and a way for particpating routers to learn about it (static, auto-rp, BSR). I must admit that I'd have to research deeper if that is also true if you only have a single mcast router.

That's also why I suggested starting with PIM dense mode - it does not require an RP, so the question and its complexities does not even come up.

I just have to enable Igmp snooping and querier on both vlans and then Ip multicast-routing and PIM sparse mode on the vlan interfaces?

Multicast routing, IGMP snooping, pim mode for interfaces: yes. IGMP querier: not necessarily. As soon as you there is an active multicast router in a subnet (actually: a router's PIM enabled interface, even if it's an SVI), this router will process IGMP, too, and will perform the IGMP queries - there is no need to define an IGMP Querier explicitely.

IGMP Querier (for IGMP Snooping) is an auxiliary construct for (V)LANs without multicast routers, to allow all igmp snooping switches to learn a (dummy) mrouter port and keep their multicast forwarding tables tidy (and not have to resort to flooding).

[/ADD-ON1]


[1] There's some Cisco catalyst switches with inter-vlan routing support that have "PIM stub" support: multicast forwarding between locally attached subnets, unable to form PIM adjacencies with other routers.

[2] the simpler dense mode might be good enough to start with.

| improve this answer | |
  • Hi thank you Marc Appreciate the reply! I am still getting familiar with Mrouter and PIM for Multicasting. in my case I don't need the rendezvous point address? cause I only have 1 layer 3 switch right? I just have to enable Igmp snooping and querier on both vlans and then Ip multicast-routing and PIM sparse mode on the vlan interfaces? – Siixth Aug 12 '19 at 0:00

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