STP LOOP GUARD / UniDirectional Link Detection

• Loop guard is enabled on L-2 interfaces only
• Unidirectional link:
• due to stp unable to receive bpdu packets, but send the packets
• cause, 
• max-age expired
• 2 switches bothe elected disignated port at same time
• Example: In case of optical channels send channel not works but receive channel is working
•  Due to layer-1 issue send channel is not working 
• then BPDUs from SW2 to SW3 not reached
• Blocked port on SW3 automatically Designated port after max-age timer expired
• solution is create L-1 keep alive, that can we send both incoming & outgoing on the same link
• 
  

Functionality	Loop Guard	UDLD
Configuration	Per-port	Per-port
Action granularity	Per-VLAN	Per-port
Autorecover	Yes	Yes, with err-disable timeout feature
Protection against STP failures caused by unidirectional links	Yes, when enabled on all root and alternate ports in redundant topology	Yes, when enabled on all links in redundant topology
Protection against STP failures caused by problems in the software (designated switch does not send BPDU)	Yes	No
Protection against miswiring.	No	Yes

Source: cisco documentation

• Commonly run both loop guard & UDLD run at same time
• spanning-tree loopguard default / spanning-tee guard loop
• UDLD
• udld port 
• udld port aggressive
•