Strange RSPAN behavior

Unanswered Question
Mar 13th, 2007


I try to set a RSPAN session but I didn't get what I expected.

My configuration:

First I set up a RSPAN VLAN (id 999) on the vtp server switch.

vlan 999



Source ports set up on a 3550 (12.1(22)EA8a)

monitor session 1 source interface Fa0/48

monitor session 1 destination remote vlan 999 reflector-port Fa0/30


Destination port on a C2940 (12.1(22)EA8a)

monitor session 1 source remote vlan 999

monitor session 1 destination interface Fa0/3


If I plug in a sniffer on port 3 of the 2940 I see broadcast, multicast and some unicast traffic. The strange thing about the unicast traffic is that it doesn't belongs to the PC which is connected to the source port.

Also if I PING the PC on the source port I didn't see the ICMP packets on the sniffer.

Any ideas?

Best regards,


I have this problem too.
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Richard Burts Tue, 03/13/2007 - 06:43


Is there a trunk between the switches that is carrying vlan 999?



MicronasSS Tue, 03/13/2007 - 06:45


Yes, there is a trunk between the switches and all VLANs are allowed on the trunk.

Amit Singh Tue, 03/13/2007 - 07:03


Have you configured " remote-span" under the vlan 999 on 2940 switch. Please verify is with the output of " show vlan "

-amit singh

MicronasSS Tue, 03/13/2007 - 07:11


both switches are member of the same vtp domain. Iconfigured VLAN 999 on the vtp server along with "remote-span".

"show vlan" on both switches says that VLAN 999 is a Remote SPAN VLAN.

MicronasSS Tue, 03/13/2007 - 08:32


I replaced the 2940 with a 2950 and it works :).

Thanks for that info.

Best regards,


MicronasSS Tue, 03/13/2007 - 08:35


I have one other question.

Do you know if the new Cisco 3540 will support this feature?

Thanks again.

Amit Singh Tue, 03/13/2007 - 08:39

3540 ????

I think you are talking about 3560 switch. Yes, this switch supports RSPAN and you will be able to configure and use it.


-amit singh

ogor Thu, 06/07/2007 - 01:09

Hi Christoph,

I have performed the same RSPAN like you but not working even on a 2950 and 4506.

Please can you give me some informations about the status of the show int vlan 999 on the 3550 ?

in which vlan have you put the sniffer port ? => vlan 999 ?!!

thanks in advance



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