Diego,
I was able to get something going with using a threshold. I first created a threshold reference watch as follows:
Name threshold
Developer ID 0xffff0000
Author ackjo04
Last Modified Time Apr 25, 2013 8:08:12 AM EDT
Model Type Rtr_Cisco
Data Type Text String
Expression "0.0.0.0.0.0.0.0.0"
Instance None
Active By Default Yes
Evaluate On Demand
Inheritable Yes
Threshold None
Then, created the watch on the octet string susing the threshold reference watch above as the threshold value as follows:
Name test
Developer ID 0xffff0000
Author ackjo04
Last Modified Time Apr 25, 2013 8:03:56 AM EDT
Model Type Rtr_Cisco
Data Type Text String
Expression atPhysAddress.#
Instance All
Active By Default Yes
Evaluate By Polling every 0 Days + 00:05:00
Inheritable Yes
Threshold Threshold violated if value != threshold.
Threshold reset if value == threshold.
No Notification
Hope this helps.
Joe