CCDP - ARCH - UDLD (UniDirectional Link Detection):

• Used where there are fibre links between switches (but can also be applied to Copper interfaces)
• Interface could be seen as Up/Up but due to a mismatch on the tx/rx pairs the comms become unidirectional 
• UDLD Normal mode error-disabled the end that detected the unidirectional state - default mode
• UDLD Agressive mode disables both ends - set it with the [agressive] switch
• Uses 15 sec hello timer
• Can be applied globally or on the interface

CCDP - ARCH - STP tools

The following tools can be used to manage STP and L2 switching loops:

• PortFast: applied to a port connecting to an end user/host. Transitions the ports straight to forwarding
• UplinkFast: Offers L2 link load balancing, up to 5 secs convergence time once a link fails
• BackboneFast: Invoked when an inferor BPDU is received on a root port or blocked port. Reduces convergence times after an indirect failure.
• Loop Guard: Stops a bridging loop by preventing an Alternate port or Root port becoming a Designated port.
• Root Guard: Protects the Root switch by preventing other switches from taking the Root role.
• BPDU Guard: Apply to PortFast enabled ports. If the port recieves a BPDU the port gets shutdown
• UDLD (UniDirectional Link Detection): Detects when one-way connection exists on a copper/fibre link. Interface moves to a shutdown state and an alarm is triggered.
• Bridge Assurance: If a port that should receive BPDU's suddenly stops receiving them the port is moved to an 'Inconsistant' state and shutdown. Prevents potential loops