Cisco UDLD configuration.

                                     UDLD-unidirectional link detection 

Most network communicate in two way or bidirectional, sometime due to software or hardware failure, traffic start moving in One Direction only and this kind of issues are normaly seen in network domain where  Fibre connections are introduced.

According to STP rules, what it generally governs is that, those switches  participating in STP topology should send and receive BPDU bidirectionally between each other to make STP out of loop, by chance If one of switch port failed and BPDU start moving in One Direction, then switch which not recieving BPDU frame may then considered something is wrong in uplink switch and it's incorrectly transitioned it's own blocked port into forwarding state and finally create huge switching loops .

In order to overcome such issue,  Cisco has developed UDLD feature to ensure  that bidirectional comnication is maintained. 

Switch where UDLD feature enabled do send out ID frame on a port and wait for the remote switch to respond back with its own ID frame. If remote switch do not respond back,  then UDLD will assumes that port is in unidirectional fault, 

By default UDLD send out ID frame in every 15 sec. But some Cisco switch do send it out in 7sec. 

UDLD operate in two mode.

>Normal mode

>Aggressive mode. 

Normal mode is the default mode and it doesn't shutdown the port instead set flag that port is operating in unidirectional mode. 

In aggressive mode - port is simply put into errordisable state,  which is auto recoverable if BPDU start recieve on the port,

UDLD can be configure globally with below command.

Switch(config)#udld enable 

Enable parameter set UDLD into normal mode,  and if you want to configure it's in Aggressive mode then you should set the parameter as

Switch(config)#udld aggressive 

And can be disabled by 

Switch(config)#udld disable 

And disabled statebcan be reset by 

Switch#udld reset 

Detail information for UDLD can be viewed by 

Switch#show udld