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New Member

Multicast Switch Requirement

Hi everyone:

If I have several servers connected to one switch, and those servers all have an application component on them that uses multicast to communicate with each other, what would the requirements of that switch have to be?

Would it have to be a L3 switch?

Can it be L2?

I am really rusty when it comes to multicast...

Thank you

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: Multicast Switch Requirement

Hi Bhavesh

IGMP snooping is by default enabled and it would just enable switch to register multicast senders and receivers against switch ports

As far as i know unless there is something to make the IGMP queries it wouldn't.

IGMP snooping is a passive thing ie. without something making IGMP queries it can't listen to any answers. So if the switch is L2 and doesn't support IGMP snooping querier function it is going to be broadcast unless you statically map entries.

The IGMP snooping querier function isn't enabled by default.

Jon

9 REPLIES
New Member

Re: Multicast Switch Requirement

Hi,

if it is not crossing same subnet, you do ot need L3. You need to closely montior the CPU utilization of L2 switch. If it is high, configuration tweaking would be required.

Regards,

Bhavesh

New Member

Re: Multicast Switch Requirement

What do you mean if its not crossing the same subnet???

New Member

Re: Multicast Switch Requirement

Same subnet means, same VLAN. If all your servers are from same IP range and

in same VLAN then you need not have L3 conenctivity or L3 configuration of multicast.

Servers would take care of IGMP.

Regards,

Bhavesh

New Member

Re: Multicast Switch Requirement

OK, Bhavesh, that makes sense. I ts the same, fo course, fo runicast forwarding. Same subnet - no routing.

So, what does a L2 switch do, by default, when it receives a packet with a multicast IP address and the associated multicast MAC-address in the headers?

Hall of Fame Super Blue

Re: Multicast Switch Requirement

ex-engineer wrote:

OK, Bhavesh, that makes sense. I ts the same, fo course, fo runicast forwarding. Same subnet - no routing.

So, what does a L2 switch do, by default, when it receives a packet with a multicast IP address and the associated multicast MAC-address in the headers?

Joe

Very good question. If you aren't using IGMP snooping with something making the IGMP queries then the switch simply floods the mutlicast traffic to all ports within that vlan.

IGMP snooping is supported on all switches L2 and L3. However IGMP snooping only works if there is something making IGMP queries because IGMP snooping simply listens to the responses to IGMP queries.

There are 2 ways to generate IGMP queries -

1) enable PIM on a L3 interface

2) enable the "igmp snooping querier" function on the switch and then the switch will generate it's own IGMP queries.

Unfortunately, not all L2 switches support the "igmp snooping querier" function so it depends on which switch you have, see this link -

IGMP snooping catalyst support

If you don't have a switch that supports it and you don't have a L3 interface you can enable PIM on then unless you want all mutlicast to be treated as broadcast you willl have to statically assign the multicast mac-address to the relevant switch ports but obviously this means you cannot have dynamic membership for the multicast stream.

Jon

Hall of Fame Super Blue

Re: Multicast Switch Requirement

bgandhi wrote:

Same subnet means, same VLAN. If all your servers are from same IP range and

in same VLAN then you need not have L3 conenctivity or L3 configuration of multicast.

Servers would take care of IGMP.

Regards,

Bhavesh

Bhavesh

Not sure what you mean by "Servers would take care of it" ? Without IGMP snooping at least the multicast would be still be treated as broadcast.

Can you clarify ?

Jon

New Member

Re: Multicast Switch Requirement

Hi Jon,

IGMP snooping is by default enabled and it would just enable switch to register multicast senders and receivers against switch ports. You are right in terms of stating that without IGMP snooping switch would treat multicast traffic as broadcast but it would still allow multicast servers and receivers to communicate.

As I mentioned earlier, it may impact only switch CPU since mac flooding will happen. Other option to prevent it is map multicast MAC address to all interfaces having same multicast group. Though it is not a recommended solution for large scale implementation.

Regards,

Bhavesh

Hall of Fame Super Blue

Re: Multicast Switch Requirement

Hi Bhavesh

IGMP snooping is by default enabled and it would just enable switch to register multicast senders and receivers against switch ports

As far as i know unless there is something to make the IGMP queries it wouldn't.

IGMP snooping is a passive thing ie. without something making IGMP queries it can't listen to any answers. So if the switch is L2 and doesn't support IGMP snooping querier function it is going to be broadcast unless you statically map entries.

The IGMP snooping querier function isn't enabled by default.

Jon

New Member

Re: Multicast Switch Requirement

gentlemen:

Interesting discussion.

I have a feeling that the default behavior of a L2 switch is to broadcast multicast packets unless it has more specific intelligence regarding who the listeners should be.

Thanks

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