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3750 Switch does not forward IGMPv3 messages (Multicast questions)


I have the following test setup in my lab


[Motorola Set Top Box (STB)] <----> [Residential Gateway (RG) ] <----> [Cisco 3750 Switch g/34]


[Cisco 3750 Switch g/44] <---> [Microsoft MediaRoom Server switch (MMR)]


When changing channels, the STB receives about 5 seconds of unicast video before it is supposed to switch to the multicast stream, however, this doesn't happen and the stream dies. Capturing in between the Cisco 3750 and the RG with a hub, I can see the IGMPv3 group join/leave messages each time I switch channels, but the IGMP messages do not appear to be forwarded to the MMR. Further investigating, I mirrored rx traffic on g/34 to g/35, but cannot see the IGMPv3 messages in the resulting capture. I have IGMP snooping, IGMPv3 snooping and report suppression turned off. g/34 is on vlan 34 and g/44 is on vlan 172. Both are configured as switchport access. I have attached my configuration below. Any and all suggestions are appreciated. Thanks!


Reza Sharifi
Hall of Fame Expert


Since you have 2 vlans, can you enable pim on both vlans and test again?

ip pim sparse-dense-mode

Also, is "ip multicast routing" enabled?



I have set "ip multicast routing distributed." When I try to set the vlan to sparse-dense-mode, I am only presented with the option "ip pim passive":

conf t

interface vlan 34

only command available is "ip pim passive"



Trying adding "ip pim passive" to both vlans and test again?


Hi Reza,

I have tried adding ip pim passive to both vlans but I am still experiencing the same behaviour as before. I understand how changing pim or igmp parameters on the switch can affect the traffic, but I am unsure why the mirror span does not show the IGMPv3 traffic going into port g/34. Does the switch modify the traffic before mirroring it onto g/35 to remove IGMP? 


Jon Marshall
VIP Community Legend

The IGMP requests will never be forwarded to the MMR because the client and server are on different IP subnets.

How does the application work ?

Does the client contact the server via unicast and then the server automatically switches over to multicast or does the server have to receive an IGMP request to switch over ?


Hi Jon,

Thanks for the response. The client sends an IGMP leave for the current channel and then a join for the channel that it wants to switch to. How can I make the Cisco switch transparent to IGMP between the subnets?

If the current channel is unicast I can't see the point of an IGMP leave ? 

My question still comes down to what I asked before ie. does the server need to get an IGMP join before it will transmit multicast or does it automatically switch to multicast ?

Because if it automatically sends the multicast stream the issue is not the server getting the IGMP request, it is because your switch is running IP Base and only supports PIM in passive mode on SVIs but there may be a workaround.

That is why I am asking the question.

If the server does have to receive the IGMP request then you would need the client and server in the same vlan.


Hi Jon,

I apologize for not answering your original question. The server must receive an IGMP join from the host before it will transmit multicast. You are correct that my switch only supports PIM in passive (no sparse-dense) mode. 


It might be that your switch does not have "IP Services" license or the correct image installed and that is why it only support PIM in passive mode.

Most newer switches need "IP services" license to support PIM.


The switch will not forward the IGMP request between vlans.

There is an "IGMP proxy" feature which I have never used and don't even know if it is supported on your switch.

Note even if you had the IP Services license and could run PIM on all interfaces this still wouldn't solve this problem.

If I get the chance I will run a quick lab but it is only an emulator so even if I can get it to work it may not work on your switch.



Okay, so to forward from one VLAN to another and vice-versa I need IGMP Proxy feature, which my switch does not support. Can you provide suggestion for switches that may meet my needs?


I did a bit of reading on IGMP proxy and I'm not sure it is what you need to be honest ie. I don't think it will solve your problem and I couldn't test it as the lab switches didn't support it

The problem is not forwarding traffic between vlans although because of your license we would still need a workaround which may or may not work.

But that is irrelevant at the moment.

IGMP only tells the switch the client wants to receive the traffic and the switch then forwards the multicast stream received from the server to the client ie. it is assumed the stream is already being transmitted.

So IGMP does not go across vlans and if the server really does need to receive the IGMP request this is the same as saying the client and server need to be on the same vlan.

Perhaps they do but I would be surprised because most network nowadays use vlans so it would be somewhat limiting to say the least.

The switch could send it's own IGMP join request but it is not recommended on a L3 switch and I can't guarantee it would work.

Multicast usually works by the source transmitting the stream and interested clients requesting the stream. 

What doesn't usually happen is that IGMP is used to trigger the sending of the stream ie. IGMP is only used to ask for an existing stream.

That said I have not used the application you are referring to so it may behave differently.