From my notes :
UDLD and BFD are designed to detect issues between devices.
UDLD is a L2 mechanism and BFD is a L3 mechanism.
UDLD is designed to detect failure on a single L2 link. BFD is designed to detect failures in between two L3 peers. This can span more than a single L2 link.
udld to stop spanning tree issues when there is an issue 1 way with the link? Does it work with stp or are they separate ?
UDLD was mainly designed to prevent layer 2 bridging loops between directly connected devices, where a switch would put a port into the forwarding state, in the case of STP for example, when it stops receiving BPDUs due to a uni-directional link. To that end, it sends echo PDUs out the port and listens for replies. There may however be other cases where having UDLD is also beneficial.
Do bfd neighbors have to be directly connected ?
Not necessarily. to be directly connected.
when and where would you use bfd?
BFD used to detect loss of communication, which will help the IGP to converge quickly.
