Duplicate multicast packets on a span session Port.

[Timothy Kleinloh]

Hello,

we are dealing with the following problem:

If I monitor a trunkport on the rootbridge in both directions  I get Duplicate Multicast Packets on the perticular VLAN.  

The first guess is, that this is worked as designed and not a IOS Bug (Platform CAT6500 SUP720 IOS 12.2(33)SXI9 )

Until know I only found an old Cisco press link from 2002 with this subject.

http://www.ciscopress.com/articles/article.asp?p=25329

I need some advice whether we are dealing with a bug here or not

- and if yes  - how I can further Troubleshoot this Problem.

Does anyone have some experience with this scenario ?

If you need any more Information please feel free to ask.

Thanks in advance for any help.

Tim

[cflory]

See here for an explanation:

https://supportforums.cisco.com/message/555658#555658

[Timothy Kleinloh]

hi there Cflory

this ist what one of the answers of your posted thread was:

Re: VLAN spanning and duplicate packets 
If you have a monitor session set up and the source is a vlan, you will get a copy sent to the span port when the packet enters the VLAN (rx) and another copy of the same packet when it exits and access port (tx).  To get around this you can set the span session to only monitor rx traffic so you only get a copy of the packet as it enters the vlan or enters an access port but not both.  This was an issue for me when setting up a monitor session for an IVR system and monitoring rx traffic only eliminated the duplicates.

Does this reaction have anything to do with that the rootbridge is local ?

Because we do not get the same reaction if we do the trace on a different Platform cat4900M .

Also I have to mention that this Problem arises only with Multicast.