Multicast - initial IGMP message from source

[Kevin SAS]

Hello guys,

I am working on some multicast project with IP camera.

This is a l2 network with all camera inside, IGMP v2 enabled with querier function configured on one switch.

When I do a show ip igmp snooping group, I have only the ports where I have receiver(s). So it means receiver have send IGMP membership report with multicast IP group inside : nothing wrong here.

SW-CSU#sho ip igmp snoo grou
Vlan Group Type Version Port List
-----------------------------------------------------------------------
150 224.16.18.126 igmp v2 Gi0/7, Gi0/8

In my imagination, I believe there was a message too from the source to say "hey, I am sending trafic on 224.16.18.126, please add me in the IGMP snooping table"

But it appears that I am wrong. I read the rfc and found nothing about IGMP message sended by the source.

Questions:

- Am I correct ? only receiver and routers are sended IGMP message but not the source ?

- The switch is able to recognize the source by the destination IP (and convert it in MAC )on the flow ?

I didn't find any command to find the source on the network.

Thanks in advance,

[julijime · Accepted solution]

Hi Kevin,

The receivers are the only ones that will send an IGMP membership report, the source on the other side will just start sending the multicast traffic into the network and in case it needs to be routed the First Hop Router (FHR) or in other words the gateway of the source will register this multicast source with another entity which is the Rendezvous Point (RP).

When the multicast traffic enters the switch it will replicate the packets and will forward them within that same VLAN, given that you have IGMP Snooping enabled the packets should be forwarded onto the ports where the receivers are connected.

With IGMPv2 I believe there is no command to see the source of the multicast traffic as IGMP does not need to register it, only if you were running PIM within the multicast network a source and group entry (S,G) will be created in the "show ip mroute" table and that's where you will see the source registered.

HTH

[Kevin SAS]

Hi @julijime

Thanks a lot for your detailled response. 

I was waiting to perform some test before to validate the post.

So I did anothers tests and saw that some IP camera are sending IGMP report for their own multicast group. And the switch is reporting that like this (capture_2). You have the sender port and the uplink port where is the receiver. 

For the other IP camera, I added the following command in order to have multicast flow on receiver

ip igmp snooping vlan 150 mrouter int gi 1/1

My problem was well explained here :

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/68131-cat-multicast-prob.html

IGMP reports were not forwarded on others switchs unless I specified manually the mrouter port.

This is an "issue" only with IGMP querier. But my switch don't have PIM.

Have a good day

kévin