Mulicast groups and membership in the group

Answered Question
Jun 19th, 2008
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How can I view a members (ports) of multicast group on Cisco Catalyst 6500 switch? I can see all multicast groups, but I don't see a members of this groups. For example:

K#sh ip igmp membership 239.192.22.171

Flags: A - aggregate, T - tracked

L - Local, S - static, V - virtual, R - Reported through v3

I - v3lite, U - Urd, M - SSM (S,G) channel

1,2,3 - The version of IGMP, the group is in

Channel/Group-Flags:

/ - Filtering entry (Exclude mode (S,G), Include mode (G))

Reporter:

<mac-or-ip-address> - last reporter if group is not explicitly tracked

<n>/<m> - <n> reporter in include mode, <m> reporter in exclude


Channel/Group Reporter Uptime Exp. Flags Interface

*,239.192.22.171 x.x.x.x 00:31:13 02:21 2A Vl11


Can I see what ports are included in the multicast group?

Correct Answer by Nagendra Kumar ... about 8 years 11 months ago

Hi,


Multicast IP to MAC convertion is done by mapping Low-order 23 bits of Multicast IP address into low-order 23 bits of 0100.5e00.0000.


For example:


224.0.0.1 = 1110 0000.0000 0000.0000 0000.0000 0001 in binary

low order 23 bits = 000 0000.0000 0000.0000 0001

hex equivalent = 0 0 0 0 0 1

result of mapping = 0x0100.5e00.0001


http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a0080094b55.shtml


HTH,

Nagendra



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n.nandrekar Fri, 06/20/2008 - 02:14
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hi!!!

The interface is seen in the command that you have specified as "Vl11" As vlan 11 is the L3 interface that you have it will figure as the interface for the igmp reports.

the traffic will be flooded on all the ports on that vlan unless igmp snooping is enabled.

If snooping is enabled you can check which ports in that vlan have recieved a igmp membership report. This is possible by looking in tha mac-address table for the multicast mac address.

eg :

sh mac-addresstable | i 0100


shows


999 0100.5e01.0101 static Yes - Gi4/1,Gi4/2,Router

which tells me that gi4/1 and 4/2 have recieved a membership report and added in the snooping table.


You can also use the command "sh ip igmp snooping stat" to know the L2 ports which are members of the multicast group.


Regards,

Niranjan

(please rate the post if it helps)


sshumakov Fri, 06/20/2008 - 02:45
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Thank you! You have shown me the right direction. Command "sh mac-address-table multicast vlan 11 igmp-snooping" gives a list of ports, which are part of the multicats groups. For example:

11 0100.5e40.16ab static Yes - Te1/1

11 0100.5e40.0cd8 static Yes - Te1/1,Gi7/26,Gi7/27


But how do you know the MAC address of the group, knowing its IP address?


P.S. The command "sh ip igmp snooping stat" does not return any result.

Correct Answer
Nagendra Kumar ... Fri, 06/20/2008 - 03:43
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  • Cisco Employee,

Hi,


Multicast IP to MAC convertion is done by mapping Low-order 23 bits of Multicast IP address into low-order 23 bits of 0100.5e00.0000.


For example:


224.0.0.1 = 1110 0000.0000 0000.0000 0000.0000 0001 in binary

low order 23 bits = 000 0000.0000 0000.0000 0001

hex equivalent = 0 0 0 0 0 1

result of mapping = 0x0100.5e00.0001


http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a0080094b55.shtml


HTH,

Nagendra



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