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