IGMP Snooping without Router

Hi Parajuli,

The multicast querier is the router with the lowest IP address. This can be confusing sometimes as the PIM designated router is the one with the HIGHEST IP address.

Rene

I have one question related to a scenario similar to the topology shown early in this article with the multicast router.

Assume you still have the source, various L2 switches as shown, and the router R1, however the receivers for the multicast receivers are actually on the far side of R1. Assume there are one or more routers connected to R1 that are not shown. The receivers we are concerned about are connected to those routers. A loopback on R1 is configured as the RP for the multicast group(s) in question.

How does SW1 know to forward the multicast traffic out Fa0/1 to R1 in this case? What configuation is required?

Is the multicast traffic inherently forwarded to the multicat router/querier?

Every article I can find either discusses how IGMP is handled in a L2 broadcast domain, or demonstrates how PIM joins enable multicast traffic to flow from the FHR. I haven’t been able to find any good explanation concerning the behavior when you combine PIM on downstream routers, and one or more L2 switches between the multicast source and the first hop router.

Hi B,

When IGMP snooping is enabled, the switch will learn about the mrouter port. Let’s say a receiver is located behind R1. Our switch will nog see any IGMP messages from hosts but it does learn the mrouter port. When the source starts streaming, the switch will forward it on the mrouter port and it ends up at the RP. The RP can then deliver the traffic to the receiver.

Hope this helps!

Rene

Thank you. That was generally what I was slowly finding.

Either with a dumb switch and/or no IGMP snooping configured the multicast traffic should be flooded out all ports.

With appropriate ASICS and IGMP snooping the mrouter port should be learned and multicast traffic flooded toward the mrouter as well as any devices that send an IGMP join for the stream.

Hi Rene, Please change the below

And let’s verify that both routers have learned the multicast router through its general queries:

to

And let’s verify that both switches have learned the multicast router through its general queries:

Thanks Sean I just changed this.

Great info - thanks!

Let’s say I have 4 VLANs (but IGMP is not needed on VLAN 300 and 400) and I want to use a switch for IGMP querier duties as pfSense 2.3.2 does not support IGMP Proxy. The bug has not been fixed.

PC 192.168.1.20 on VLAN 100 is a video stream. Will PCH 192.168.2.20 (different subnet) on VLAN 200 be able to see and report back to this multicast if VLAN querier is set on both VLAN 100 and 200?

Thanks!

Hi Vilhelm,

IGMP Snooping (querier) is used for multicast traffic within the VLAN. If you want to get multicast traffic from one VLAN to another, you will have to route it. You’ll need a device with multicast routing enabled.

Rene

Hi, if etherchannel is used to trunk the 2 switches, does all the multicast traffic “flood” the trunk when the IGMP querier is enabled - basically the switch automatically makes the trunk an mrouter port and forwards all multicast traffic.

This is what I’ve been told at least. We are trying to do very high bandwidth, high quality video streams (a kind of IPTV config), and I’m told that the 80gpbs trunk link is being saturated by the many (75 channels) of 8Gbps (4k uncompressed) video streams we have on the network.

This is Layer 2 only, we don’t have Routing licenses to enable PIM-SM.

We are not Cisco experts, and this is happening at a customer site.

thanks

Hello Vanya,

Your traffic should only be flooded if your switches don’t have a “router port” for multicast in your topology. This might depend on your switch model / IOS version though. Most switches will drop membership reports and flood your multicast traffic.

Adding snooping querier to your interfaces adds a “router port” to the topology so it solves the problem of flooding, not causing it.

Do you have multiple VLANs? Is it enabled on each VLAN?

SW1#
interface vlan 1
 ip igmp snooping querier

There is no need to enable PIM on any interfaces in your case, you only need this if you want to route multicast traffic between subnets. IGMP snooping querier is enough in a L2 topology.

Rene

For IGMP snooping without a router, it states that “H2 will not receive anything. SW2 will receive the IGMP report from H2 but doesn’t know a router port, so it doesn’t forward the IGMP report towards SW1.” But in the paragraph above it states “switches are flooding all multicast traffic now, treating it like broadcast traffic.” If it’s flooding all ports, then it would seem to me that SW2 would receive the traffic and then in turn H2 would. Please explain why it doesn’t quite work that way. Thank you.

Hello Mike,

It depends on the switch / IOS that you use. In my case, I used 3560s and those drop the IGMP reports and just flood multicast traffic so in that case, H2 does receive the traffic.

Other switches might accept the IGMP report and not broadcast multicast traffic when they don’t have a router port. In that case, H2 will never receive anything since SW2 doesn’t know where the router port is…it can’t forward the IGMP report to SW1.

Rene

Hi,
I have a question using Multicast without a Router. The topology with the ip addresses is attached.
Some questions:
Is this Setup Working? (due to different Subnet)
I assume Yes , because of IGMP snooping, SW2 and SW3 knows where to forward the Multicast traffic.
Is the main reason für a multicast router to have a querier in the network ?
Thanks a lot in advance

Best regards , FlorianMulticast

Hello Florian

Rene has a lesson dedicated to this very question. You can find it here.

However, to provide a quick answer to your questions:

Yes it should work. In Rene’s lesson, he provides to solutions to making multicast work without a router. One is to make one switch a querier. The other is to configure PIM on the SVI interface of one of the switches. Take a look at the lesson for more details.

Essentially, yes, this is one of the fundamental functions of a multicast router.

I hope this has been helpful!

Laz

Hello Laz,
thanks for your reply and your answers. I already studied your mentioned lesson before my post. Main difference between the lessons architecture and mine was, that I had two separate Subnet, whereas in the lesson all equipments were located in the same subnet.

Best regards, Florian

1 Like

Hi NL Team,

Does the attached look like a “healthy” debug from an IGMP debug. This was taken from the “querier”. It is a multicast environment with no Layer 3 router as such. I see there seems to be V2 and V3 messages also. Is there anything there that looks of immediate concern?

Output.txt (7.9 KB)

Hi Robert,

Looks OK to me. The “Report has illegal group address 224.0.0.113” messages that you see are normal. The 224.x.x.x range is link-local multicast so these are ignored.

Rene

Hi

I have a problem in real life with igmp. I have two host that send imgp version 3 between two diferent interfaces VLAN (SVI) they need to see igmp version 3 each other. With the command ip igmp snooping querier will hosts see the igmp version 3 eventhough they are in separate VLANs and broadcast domain?

Thank you!!

Hello Carlo

If you have a setup where you have two VLANs and you require routing between them, then IGMP Snooping without a Router is not a solution for this. These solutions are only for scenarios with a single VLAN.

The ip igmp snooping querier command will make the SVI port of a switch a querier, but it will not be able to route IGMP messages to other VLANs, but the will remain only within the specific broadcast domain. If you have such a scenario, it is better to go for using a multicast router since you need the routing functionality.

I hope this has been helpful!

Laz

1 Like

Thank you !
This works