There is a monitoring template applied to the Interface. If you click on an interface on the right side in the middle of the display you will see a drop down. Click that and choose "Templates" to see the template that is assigned to the Interface.
Components (interfaces, filesystems, etc.) are discovered at modeling time and the appropriate template is automatically bound to them. Component templates are easily distinguished becase they have a 'snow flake' icon next to them. Those templates you do not bind yourself and Zenoss handles it during the modeling process.
You would need to modify the interface template to add the threshold you are looking for. Most likely you have a template that is defined at /Devices that got attached to that component. If you wanted that template to affect only your Cisco switches you could override that template at the Device class level and then modify that template.
You can go to Advanced -> Monitoring Templates and find the template (look at the template that was attached using the instructions in my first paragraph). You can then click the "Gear Menu" and choose "Override/Copy Template" and choose the Device class that you have your cisco switches in (/Network/Switches/Cisco for example).
Now any device that you add to that Device class will use this interface template for it's interfaces (assuming its interfaces are discovered to be the same type).
--Dennis
