Multicast - Making a host reply to a multicast group address ping?

[thelastone]

Hi fellow networkers,

   I am practising on Multicasting, specifically  Sparse-Mode. One command I used is the typical "ip igmp join-group  239.99.99.99", which Layer 3 interfaces will reply when I do a ping to  this multicast group address. I have a working Multicast Source (VLC  Player performing the streaming), as well as a working Receiver (VLC  Player receiving in another host), which the Receiver is able to see the  video and "show ip mroute" is showing all the multicast routes  correctly.

   I spent many days of searching and testing but  still don't have an answer to my question, which is as follows: After I removed the   "ip igmp join-group 239.99.99.99", but still keeping the Multicast Receiver to see the video, why can't I see any response when I ping to  the multicast group address? Since the Multicast receiver is able to  see the video, it is still joined to the multicast group but why didn't  it reply to a multicast group ping? Please kindly enlighten me. In  addition, is there any way that I can make the host reply to the  multicast group ping as I just do not want to use the "ip igmp join-group"  command.

Many thanks in advance,

Steve

[Peter Paluch · Accepted solution]

Hi Steve,

One command I used is the typical "ip igmp join-group  239.99.99.99",  which Layer 3 interfaces will reply when I do a ping to  this multicast  group address.

What this command really does is make the CPU inside your router to join this group itself. In other words, the router itself becomes a member of this group that receives and processes the packets addressed to 239.99.99.99. That makes it respond to pings sent to 239.99.99.99 along with forwarding them along the multicast distribution tree.

After I removed the   "ip igmp join-group 239.99.99.99", but still  keeping the Multicast Receiver to see the video, why can't I see any  response when I ping to  the multicast group address?

My assumption is that the ping responses were sent by your router. Current host operating systems like Windows or GNU/Linux do not respond to multicast or broadcast pings. Therefore, after you removed the ip igmp join-group command from your router, it stopped being a member of this group, and being the only member that actually responded to multicast pings, there was no station to respond to your pings.

Your pings most probably reached the stations even after removing the ip igmp join-group command but as explained earlier, host OSes do not respond to multicast/broadcast pings so that was the reason you didn't get any response back.

I do not think Windows can be persuaded to respond to a multicast ping. You will have to google for it. Under GNU/Linux, responding to broadcast/multicast pings can be activated by issuing the command

echo 0 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

Please feel welcome to ask further!

Best regards,

Peter

[Giuseppe Larosa]

Hello Steve,

as Peter has explained pinging to the multicast group and receiving an answer has to be seen as a troubleshooting aid provided by Cisco routers that have an igmp join configured.

When the multicast receiver on the host PC receives the multicast video stream you can easily demonstrate that everything is going well even if the receiver is not responding to ICMP echo request sent to the multicast group.

Just for example a

show ip mroute active

on network devices should give enough info

Be also aware that in production networks is really unsafe to leave igmp join-group commands on network devices as it expose them to very high cpu usage if the traffic volume of the multicast stream is high (the traffic is process switched sent to the main cpu).

Hope to help

Giuseppe

[thelastone]

Thanks Peter and Giuseppe for the answers. I now understand that responses to multicast pings on the host side is done at the OS level. I had incorrectly thought that a VLC Player joining the multicast group would respond to the multicast pings. I'm very grateful for your responses!

Steve

[thelastone]

Hi again guys, I have one more question - If I have a CME that is configured with a MOH stream to 239.99.99.99 but not using any "ip igmp join-group" command on the CME, will there be any ping response when I ping to 239.99.99.99?

I'm not a voice expert, which I cannot get MOH to work correctly in my lab, so would like to ask your help in my query above. Thanks alot!