I was running some IPS task using RSPAN and hit an obscure 12.55(SE) bug -
thought I'd share it with you folks to that you don't waste time on this:

The symptom is your dot1q trunk will go into PVID inconsistent state for the
source VLAN and RSPAN VLAN.
The PVST BPDUs of the source VLAN are carried in the RSPAN VLAN and this
confuses the receiving switch.

This is bug  
CSCtj76880.<http://cdets.cisco.com/apps/dumpcr?&content=summary&format=html&identifier=CSCtj76880>

Cat 3 - 3560 12.2(55)SE1

vlan 1122
 remote-span

monitor session 1 so vlan 12
monitor session 1 dest remote vlan 1122

Cat 4 - 3560 12.2(55)SE1

vlan 1122
 remote-span

monitor session 1 so vlan 12 , 1122
monitor session 1 dest int Fa0/15

Cat 3 and Cat4 are dot1q trunked.

The bug will manifest as a PVID inconsistent state on the trunk link for
vlans 12 and 1122.

The resolution is to use ISL trunking, 12.2(58)SE (not available on older
switch models), or revert IOS version.


Richard
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