Who will reply when pinging a multicast IP address?

Basically all the hosts joined the multicast group will reply the ICMP request

Below is an example using the RIPv2 protocol:

Basic configurations of RIPv2 were set up on the interfaces with each router having it's router number as thelo0interface address (i.e. R1 -1.1.1.1). For verification, here's the rip routing table on R1.

R1#show ip route rip
     2.0.0.0/32 is subnetted, 1 subnets
R       2.2.2.2 [120/1] via 10.1.1.2, 00:00:07, FastEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
R       3.3.3.3 [120/1] via 10.3.3.3, 00:00:26, FastEthernet0/1
     10.0.0.0/24 is subnetted, 3 subnets
R       10.2.2.0 [120/1] via 10.3.3.3, 00:00:26, FastEthernet0/1
                 [120/1] via 10.1.1.2, 00:00:07, FastEthernet0/0
R1#

When pinging the multicast address of224.0.0.9, 3 replys came back;R2sent a reply,R3sent a reply, andR1replied to itself because it is also listening for that address.

R1#debug ip icmp
ICMP packet debugging is on
R1#ping 224.0.0.9 source lo0 repeat 1

Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 224.0.0.9, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1

*Mar  1 00:15:41.163: ICMP: echo reply sent, src 1.1.1.1, dst 1.1.1.1
*Mar  1 00:15:41.163: ICMP: echo reply rcvd, src 1.1.1.1, dst 1.1.1.1   <--- R1 Response
*Mar  1 00:15:41.179: ICMP: echo reply rcvd, src 10.1.1.2, dst 1.1.1.1  <--- R2 Response
*Mar  1 00:15:41.179: ICMP: echo reply rcvd, src 10.3.3.3, dst 1.1.1.1  <--- R3 Response
Reply to request 0 from 1.1.1.1, 8 ms
Reply to request 0 from 10.3.3.3, 24 ms
Reply to request 0 from 10.1.1.2, 20 ms

Pinging a multicast address will generate a response from participants listening to that address.