This is something I ran into during my multicast studies and I’m sure I’m not the only one who was thinking this…

So let’s say we have the following topology…

mcast_topo

 

For those who were at my Multicast meetup May 31, 2014 — this topo will look very familiar to you! Anyhow, so lets say we have our Source router on the left sending the traffic towards the Receiver router on the right. You may notice we get this output when we do pings to the mcast group.

Source#ping 239.1.1.1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.1.1.1, timeout is 2 seconds:

Reply to request 0 from 192.168.61.10, 172 ms
Reply to request 0 from 192.168.61.10, 172 ms

So why do I get multiples replies here? I’ve yet to get a 100% confirmation answer on this but it appears to be the number of PIM enabled interfaces. See below that I have PIM enabled on two interfaces off the source router. If I remove the PIM statement from the loopback, notice that I now only get 1 reply.

Source#sh ip pim int
Address          Interface                Ver/   Nbr    Query  DR     DR
                                          Mode   Count  Intvl  Prior
192.168.12.1     FastEthernet0/0          v2/S   1      30     1      192.168.12.2
1.1.1.1          Loopback0                v2/S   0      30     1      1.1.1.1

Source#ping 239.1.1.1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.1.1.1, timeout is 2 seconds:

Reply to request 0 from 192.168.61.10, 172 ms

So my guess is that this is the number of interfaces PIM is enabled on the router sourcing the traffic. Anyone else find anything different but this seems solid for me.

 

William Zambrano

William Zambrano

NYC networkers is run by William Zambrano, a passionate network engineer who has been in the IT industry for eight years who posts up blog articles, YouTube videos, and holds meetup.com events in the NYC area. He lives in Queens, New York and has consulted in various different companies in the NY area. Previously William worked as a Cisco Certified Systems Instructor (CCSI) but now currently works for Arista Networks serving as a Systems Engineer. William can be reached by email at willzambrano@gmail.com

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