Duplicate messages in mulitcast test

Question

We are doing a multicast test on the multicast IP=224.0.0.115 port=61101. But we are getting some duplicate messages as given in the test output.
This is done from host sjh75a01,sjh75a02,sjh75a03,sjh75a04,sjh75a05
sjh75a01 neop04 $ java utils.MulticastTest -N sjh75a01 -a 224.0.0.115 -p 61101 -s 5
***** WARNING ***** WARNING ***** WARNING *****
Do NOT use the same multicast address as a running WLS cluster.
Starting test. Hit any key to abort
Using multicast address 224.0.0.115:61101
Will send messages under the name sjh75a01 every 5 seconds
Will print warning every 600 seconds if no messages are received
I (sjh75a01) sent message num 1
Duplicate message from sjh75a01: 1
New Neighbor sjh75a02 found on message number 1
New Neighbor sjh75a03 found on message number 1
New Neighbor sjh75a04 found on message number 1
New Neighbor sjh75a05 found on message number 1
I (sjh75a01) sent message num 2
Received message 2 from sjh75a01
Duplicate message from sjh75a01: 2
Received message 2 from sjh75a02
Received message 2 from sjh75a03
Received message 2 from sjh75a04
Received message 2 from sjh75a05
we have Weblogic servers on 5 physical hosts that need to broadcast messages to each other via IP multicast (IP=224.0.0.115 port=61101). These broadcast messages are used by WebLogic servers to update their cluster membership and are hence very important.
The test results indicate that each host receives packets sent out from all other 4 hosts once, but receives the packet that it sent out twice.
For example, if host A and host B send out packets "a" and "b" respectively to the multicast IP/port, then host A receives packet "a" twice (duplicate) and packet "b" once (normal). Similarly host B receives packet "a" once (normal) and packet "b" twice (duplicate).
We wish to know if this a normal behaviour

Answer 1

Hi Deepak,
I can say from my personal experience that whenever we use same multicast ip address and port for the purpose of multicast utility then we will get the duplicate messages.
If that is not the case, then I can say the following are the reasons for duplicate packets and it is better if we can contact your network team to check the following :
***Duplicate multicast packets are symptoms of several things:***
1. An ethernet-level device (e.g. bridge) that doesn't understand multicast so reforwards it. This is easy to track to the right subnet using mtrace's packet statistics, but hard to track to the final device because it doesn't change the ethernet source address.
2. The design of the PIM protocol accepts multiple copies periodically; when there are multiple paths to a multi-access link the default state is that all routers on the link forward; PIM Assert messages elect a single router as the forwarder but periodically time out.
3. Forwarding loops. This is often due to a misconfigured router.
Please cross verify the same but I am agreeing partly with Jay as most of the times we consider that the multicast test is passed even if there is any duplicate messages. But reconsider the above reasons and trace it out along with your network team..
Thanks,
Sreedevi