udld...

smoke em if you got em...

dls1(config)#int rang g0/1 - 2                                                           
dls1(config-if-range)#sw trunk encap dot1q                                               
dls1(config-if-range)#sw mode trunk                                                      
dls1(config-if-range)#no shut  

dls1(config)#udld enable                                                                  
dls1(config)#int rang g0/1 - 2                                               
dls1(config-if-range)#udld port aggressive    

unidirectional link detection...

when bidirectional links go uni, udld will detect this behavior and shut the link down...                           
                                                                                        
dls1#sh udld g0/1                                                                        
                                                                                         
Interface Gi0/1                                                                          
---                                                                                      
Port enable administrative configuration setting: Enabled / in aggressive mode           
Port enable operational state: Enabled / in aggressive mode                              
Current bidirectional state: Unknown                                                     
Current operational state: Advertisement                                                 
Message interval: 7                                                                      
Time out interval: 5                                                                     
No neighbor cache information stored                                                     
dls1#

aggressive means after eight failed  attempts to prove the link is yet bi, the port will be errdiabled

note that sh udld g0/1, does not use interface as an argument...       

                                           i prefer bidirectional... links