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New Member

Monitor Multicast Users

Dear All,

I have been using the command:

show ip igmp membership

to monitor how many users are tuned into a boradcast stream. However, to my disappointment, I have realized that more users than the ones that were showing up with this command, were actually receiving the stream.

Does anybody know a  better command to show how many LAN Ports are connected to each Multicast Grup?

Many thanks,

Francesco

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: Monitor Multicast Users

Francesco

The "sh ip igmp membership" command shows which hosts responded to an IGMP query sent by PIM on the L3 vlan interface. This won't show all the hosts who are receiving the stream as the L3 device only needs to know whether there are any hosts on a particular vlan that want the stream.

The "sh ip igmp snooping groups vlan dynamic" shows on which ports the switch has seen requests for the stream and it should show all those ports. 

Bear in mind with the first command it is done on a per vlan basis so if you had 5 switches for example all interconnected in that vlan it would not show all hosts that requested the stream because of what i described above.

The second command is switch specific so if you had the same 5 switches you would need to run this command on all those switches to get the full list of ports.

Jon

12 REPLIES
Hall of Fame Super Blue

Re: Monitor Multicast Users

Francesco

You can look at the mac address table on the switch. The multicast group address will map to a multicast mac address (and you can work it out from the group address- see link below) and see which ports are mapped to that mac address.

It does, obviously,  mean looking at each switch that the stream is being sent through.

Because of the way multicast IP addresses are converted to mac addresses it means that there are multiple multicast IPs to the same mac address but it is unusual for this to be a problem as long as you are aware of it and pick appropriate IPs.

Here is a link to convert a multicast IP to it's corresponding mac address -

http://nettools.aqwnet.com/ipmaccalc/ipmaccalc.php

Jon

New Member

Monitor Multicast Users

Thank you Jon,

but it doesnt seem to work.

I even joined the multicast from my own machine and watched the MAC addresses associated to the port on the switch I am connected to. And I didnt see any multicast MAC there, just my own.

Or shuold I use a different commadn than show mac address-table?

Thank you

Francesco

Hall of Fame Super Blue

Monitor Multicast Users

Francesco

1) do you see the multicast mac address anywhere in the mac address table ?

2) i assume the switch is running IGMP snooping (it is by default) but in addition are you -

a) either running PIM on a L3 interface

or

b) if not a) are you running an IGMP snooping querier function

Jon

New Member

Monitor Multicast Users

The multicast groups I am using are : 239.1.1.1 and 239.1.1.2 which converted are:

01:00:5e:01:01:01 and

01:00:5e:01:01:02

These MAC I can not see anywhere with the show mac address-table even while watching the multicast stream.

I can however see the IGMP Groups, but not for all hosts.:

show ip igmp membership

Channel/Group                  Reporter        Uptime   Exp.  Flags  Interface

*,239.1.1.1                    10.104.37.58    01:30:39 02:47 2A     Vl110

*,239.1.1.2                    10.104.37.254   01:51:35 02:46 2A     Vl110

a couple of hosts joining hte IGMP group appear,  but I am sure there are much more.

so the ip igmp group membership either doesnt work, or the other hosts are not using IGMP join. But at this point the question is, how can they still work, without using IGMP join group?

Francesco

Hall of Fame Super Blue

Monitor Multicast Users

Francesco

Can you answer these questions -

1) is the mutlticast source in the same vlan as the receivers ?

2) if it isn't have you enabled multicast routing with PIM ?

3) what switch model and IOS version are you running

note that if it is all in the same vlan and you are not running the IGMP snooping querier your multicast is probably being treated as a broadcast ie. it goes to all hosts.

Jon

New Member

Monitor Multicast Users

1) is the mutlticast source in the same vlan as the receivers ?

no, the source is on a different SUBNET / ROUTER / VLAN.

2) if it isn't have you enabled multicast routing with PIM ?

Yes, in fact the service is working, and some device appear in the IGMP Multicast Group

3) what switch model and IOS version are you running

The switch clients are connected to is a 3750 ver 12.2(53r)SE

The Server, as said above, is 4 Subnets away and the TTL is set to 10.

Francesco

Hall of Fame Super Blue

Monitor Multicast Users

On the switch you are not seeing the multicast mac address can you post the output of -

"sh ip igmp snooping groups vlan dynamic"   where x is the vlan with the clients in.

Jon

New Member

Re: Monitor Multicast Users

Here it is:

11        239.1.1.1                igmp        v2          Gi1/0/21, Gi3/0/6

11        239.1.1.2                igmp        v2          Gi2/0/14, Gi4/0/21

I filtered out other VLANs and other Multicast Groups.

The marked interfaces are the ones that also show up with show ip igmp membership

Thank you,

Francesco

Hall of Fame Super Blue

Monitor Multicast Users

Francesco

Are you sure there are other hosts registering for that stream because the above output shows which ports the group is being seen on.

Jon

New Member

Re: Monitor Multicast Users

I know, it's strange.

but they confirmed to me a much higher number of people were tuned onto the stream while I was performing the show command.

just to make sure, what is the difference between the command I have been using:

show ip igmp membership

and the command you asked me to do:

sh ip igmp snooping groups vlan dynamic

From the first I only see 2 IPs, while the latter gives me 4 IPs

The reason I am asking it is because they seem to me they shuold give the same result... i.e. same number of IPs tuned onto the stream at the given time.

Thank you,

Francesco

Hall of Fame Super Blue

Re: Monitor Multicast Users

Francesco

The "sh ip igmp membership" command shows which hosts responded to an IGMP query sent by PIM on the L3 vlan interface. This won't show all the hosts who are receiving the stream as the L3 device only needs to know whether there are any hosts on a particular vlan that want the stream.

The "sh ip igmp snooping groups vlan dynamic" shows on which ports the switch has seen requests for the stream and it should show all those ports. 

Bear in mind with the first command it is done on a per vlan basis so if you had 5 switches for example all interconnected in that vlan it would not show all hosts that requested the stream because of what i described above.

The second command is switch specific so if you had the same 5 switches you would need to run this command on all those switches to get the full list of ports.

Jon

New Member

Re: Monitor Multicast Users

Thank you Jon,

it makes perfect sense and it shows why I could see only 2 entries at most, which indicates one  per Multicast Group.

That explains why we couldnt see all users actually connected to the multicast.

Many thanks again,

Francesco

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