Relevant Log messages
Feb 1 03:39:08.338 EST: %IPV6-4-DUPLICATE: Duplicate address FE80::212:44FF:FE4D:4400 on GigabitEthernet8/20
Feb 1 03:39:08.904 EST: %UDLD-SP-4-UDLD_PORT_DISABLED: UDLD disabled interface Gi8/20, transmit/receive loop detected
Feb 1 03:39:08.904 EST: %PM-SP-4-ERR_DISABLE: udld error detected on Gi8/20, putting Gi8/20 in err-disable state
When you see "Duplicate address" and "transmit/recieve loop detected" on the same interface, that's almost a slam-dunk that this is a cabling problem.
Looped circuits are very common when dealing with WAN providers. It typically happens because some technician is testing the circuit with a loopNote 1 at their cross-connnect and gets pulled off to perform some other task. However, the loop prevents your circuit from coming up on both sides.
The switch is complaining because it (correctly) considers it a problem when it sees it's own packets received on the interface it was sent from. The best thing to do is work with the provider and tell them you've detected a loop at this location.
If the provider resists (not uncommon), ask them to perform a loop at on their panel at this site towards the other site. If the circuit is indeed correct, you should see your other switch link up (assuming you don't have UDLD on it, as you stated).
After the loop is corrected, just shut/no shut this port to test it again.
This problem illustrates a good use-case for out-of-band connections to your WAN sites through another carrier. This way, if you need to diagnose your far-end circuits, you've got some way to connect to your far-end router / switch.
End Notes
Note 1 A loop is just a physical or logical jumper, which connects the Transmit side to the Receive side of the same circuit. This is usually done so a tech can validate the circuit is functional to some demarc point in their network. Ironically, it takes down all connectivity past the loop (until it is removed).
Click here to view Ron's comment. Here is what he said:
Signaling is the method by which you define what a one or zero is.
Encoding is how you use the 1s and 0s to represent information.
Just posting it as an answer since Ron didn't.
Best Answer
The terms do get thrown about quite a bit and the definition is a bit of a murky issue as they can be used in several ways.
I would say that generally a leased line is a line you are paying someone else for use. For instance, you can string your own copper and establish your own T1/T3 service between buildings. However, you could also pay someone to provide and maintain that line. Often leased lines have a SLA associated with them.
A dedicated line would be a line that is dedicated to you/your company. For instance, in an office building, you may not have a dedicated line. You may have a PRI that comes into the building and provides service to multiple organizations. Often this is cheaper because the telecommunications company doesn't need as many lines going into the building and has fewer to maintain.