What do you mean by: 'but I want to monitor their secondary internet connection. ' -- what are you trying to find out.
[Excuse the poor windows Paint picture, but I work a lot better with pictures in cases like this]
if you want to know when traffic on A instead of B then I would put a custom property on the interface A, and then check to see when the traffic was significant on interfaces labeled 'A'
[or 'secondary Internet'] -- in our case we drive this from our CMDB.
If you want to know when 'A' is unavailable then the first case is when you lose link-state (i.e. interfaceA is down). we normally run a VPN connection over our link-A type connections but if there is no routing from your provider (link state=up, but traffic does not go anywhere) then you have to look at the VPN state, and I've not had much simple success with that. [actually we run OSPF over the VPN connection and alert on the routing state, but now we're deep into whatever your configuration is and you'll have to think about that some more]
Yes your pic is accurate. ADTRAN Router (MPLS) Connected to SonicWall on X2
Secondary Guest Wifi (normal broadband connection ATT, COMCAST, etc) connected to X1
I have monitoring set up to alert on when the X1 interface is "down" but the problem with monitoring on the specific interface is some of our remote sites have had bad ports and their connection is on X3 for example, or the site doesn't have guest wifi so the X1 interface show down since it's is not used. I am trying to figure out a way to monitor these connections dynamically.