ip igmp snooping on the Switch

[umetsu]

My understanding is below.

Regarding a "ip igmp snooping",if I enable it on the Switch,multicast traffic streams to the port that the mulicast receiver is connected. If disable it,multicast streams to the "all ports" like a broadcast.

Is it right ?

If it is right,I think that I can confirm the "igmp packet" on the Switch. However,I couldn't capture the igmp packet at all. Is it only the situation that the router is connected to the Switch whick I can confirm the igmp packet ?

Then, I confirmed the multicast packet on the all ports on the Switch. I'm not sure about the merit of the igmp snooping on the Switch.

I'm thinking about the following network.

multicast sender-----Switch------multicast receiver

igmmp enable

[Kevin Dorrell]

I'm not sure I understand your argument here. Your first paragraph is absolutely correct - that is how it works.

I was not so clear about the second paragraph. The IGMP is sent out by machines that want to be part of the multicast group. It is not acknowledged at all. The IGMP is flooded to all ports to say "Hey, I am someone listening or transmitting this multicast address." The switch observes these, and sends the multicast stream only to the ports where it has seen the IGMP.

It is not necessary to have a router for IGMP to work correctly. It works OK on a single layer-2 switch. Essentially, IGMP was originally designed as a protocol between hosts on a local network.

In your diagram, the multicast sender is also sending IGMPs, and so is the multicast receiver. The switch joins together those ports as far as the multicast stream is concerned. Other switch ports are excluded from the multicast traffic unless their hosts also generate an IGMP.

Is that OK?

Kevin Dorrell

Luxembourg

[umetsu]

Kevin,Thanks for your reply.

I understood your answer. But,I want to make it clear the following things more.

For example,

on the Switch,I enabled the "ip igmp snooping".

The multicast sender is connected to Port1 and

multicast receiver is connected to Port2.

Then,I connected the LAN analyzer to Port3.

These ports are members of VLAN 1.

1)Sender:ip address 192.168.1.1/24 and sends

239.0.0.100

2)Receiver:ip address 192.168.1.2/24 and receives 239.0.0.100

On the LAN analyzer,I only captured the packet of "Source:192.168.1.1" and "Destination:239.0.0.100".

Judging from this, Port3 receives the multicast packet.This result means that igmp snooping is NOT working well.Why ?

1)I think that the multicast packet should stream "only" between Port1 and Port2.

In this situation,if I mirror the Port1 and Port2 to Port3,can I capture the packet of "igmp" ?

I tried above but the result was same as the 1).

(I couldn't capture the igmp packet.)

Is it right ?

Then,how can I confirm whether the igmp snooping is working well or not on the Switch ? Is there good command to confirm ?

Very sorry to write a lot of queries.

Please let me know asap.

[Kevin Dorrell]

Never mind about the loyt of queries - your experience is valuable to me too.

As far as I understand it, on port 3 you should only see the IGMP queries from the multicast sources. You should not be seeing the multicast stream itself. (Don't forget that this IGMP query is itself a multicast from 192.168.1.1 to 239.0.0.100.) The source is saying, I've got something to say to group 239.0.0.100 - now, who is listening?

Now your receiver on port2 will say "I'm listening". I'm not sure whether that report is sent to all ports, or just to those where the switch knows there is a multicast sender. Your experiment seems to say the switch only gives it to the senders.

If you sent an IGMP report into port 3, then your analyser would get the full blast of the multicast stream. But I guess it is only seeing the IGMP query from the source - which is all it needs.

So - that packet you see on port 3 - is it an IGMP query to 239.0.0.100 from the sender 192.168.1.1?

Kevin Dorrell

Luxembourg

[dalara]

I have a question with IGMP snooping, i am using it with PIM sparse dense node on a 4500 switch,

In the output VLAN i have "no ip igmp snooping" command and sometimes i not receive the multicast video in the receiver. i only enable ip igmp snooping and the receiver start to show the video information, then i disable ip igmp snooping "no ip igmp snooping "again and continues sending the information..

Do you have some idea what is hapenning?

Thanks

[umetsu]

I tried the test on the Cat3550 as a Layer2.

Mulicast address:224.1.2.3

The mulicast sender is connected to port1.

ip address:192.168.1.1/24

The mulicast receiver is connected to port2.

192.168.1.2/24

I checked the mac-address table on Cat3550 by "show mac-address-table" command but I could NOT find the mac-address of 0x0100.5e01.0203(224.1.2.3).

I only found the mac-address of sender's NIC mac address on port1 and receivers's on port2.

Of cource,I enabled the ip igmp snooping on Cat3550.

Why ?

Please let me know asap.

[umetsu]

I understood.

Thanks for your reply.

[sbhadrav@cisco.com]

It may be more to do with what is connected on F0/17. Is it a switch? And does it do IGMP snooping? If so, it needs to know that it is talking to a multicast router so that it can potentially open all streams on that port. It does so either by static configuration on the switch, or by detecting the presence of the router by PIM hellos.

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12120ea2/2950cr/cli1.htm#wp3395441

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12120ea2/2950cr/cli1.htm#wp1365537