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How pinging a multicast address is handled ?


Here is a list of well know router protocols and their multicast addresses:

  • 224.0.0.1 all hosts on a subnet

  • 224.0.0.2 all routers on a subnet

  • 224.0.0.4 Distance Vector Multicast Routing Protocols (DVMRP)

  • 224.0.0.5 OSPF routers

  • 224.0.0.6 OSPF designated routers

  • 224.0.0.9 RIP Version 2 routers

  • 224.0.0.10 EIGRP

  • 224.0.0.13 Protocol independent Multicast (PIM)

Refer the above diagram and execute the following command :-

Send a single ping request to 224.0.0.1 ping -c1 224.0.0.1

ping output:

PING 224.0.0.1 (224.0.0.1): 56 data bytes

64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.041 ms

--- 224.0.0.1 ping statistics ---

1 packets transmitted, 1 packets received, 0.0% packet loss

round-trip min/avg/max/stddev = 0.041/0.041/0.041/0.000 ms

tcpdump output:

14:55:59.003571 00:00:0a:01:00:21 > 01:00:5e:00:00:01, ethertype IPv4 (0x0800), length 98: (tos 0x0, ttl 1, id 4340, offset 0, flags [none], proto ICMP (1), length 84)

10.1.3.1 > 224.0.0.1: ICMP echo request, id 5644, seq 0, length 64

14:55:59.003962 00:00:0a:01:00:08 > 00:00:0a:01:00:21, ethertype IPv4 (0x0800), length 98: (tos 0x0, ttl 64, id 4941, offset 0, flags [none], proto ICMP (1), length 84)

10.1.3.254 > 10.1.3.1: ICMP echo reply, id 5644, seq 0, length 64

14:55:59.003980 00:00:0a:01:00:23 > 00:00:0a:01:00:21, ethertype IPv4 (0x0800), length 98: (tos 0x0, ttl 64, id 4334, offset 0, flags [none], proto ICMP (1), length 84)

10.1.3.3 > 10.1.3.1: ICMP echo reply, id 5644, seq 0, length 64

14:55:59.003982 00:00:0a:01:00:22 > 00:00:0a:01:00:21, ethertype IPv4 (0x0800), length 98: (tos 0x0, ttl 64, id 4334, offset 0, flags [none], proto ICMP (1), length 84)

10.1.3.2 > 10.1.3.1: ICMP echo reply, id 5644, seq 0, length 64

14:55:59.003984 00:00:0a:01:00:13 > 00:00:0a:01:00:21, ethertype IPv4 (0x0800), length 98: (tos 0x0, ttl 64, id 4635, offset 0, flags [none], proto ICMP (1), length 84)

10.1.3.4 > 10.1.3.1: ICMP echo reply, id 5644, seq 0, length 64

Discussion:

Frame 1 is the ICMP echo request to IP address 224.0.0.1 which maps to hardware address 01:00:5e:00:00:01

All host/routers give an ICMP echo reply.

We see that the ping output that the first reply is from 127.0.0.1.

Conclusion

All hosts are members of the multicast group with address 224.0.0.1 and so all respond to the ping request from hostC1 to this group.


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