Configured UDLD operation mode for RFC5171. In normal mode, once the link is determined to be in unidirectional state (after it was deemed bidireccional), and no "Echo" is received, it is set to "undetermined" state. However, if a a UDLD packet with an empty "Echo" is received the interface will be set to "errDisabled". In aggressive , once a link is determined to be in unidirectional state (after it was deemed bidirectional), and no "Echo" is received, it is set to "errDisabled" state. The interface will also be set to "errDisabled" if a UDLD packet with an empty "Echo" is received.