1.2.b UDLD:
UDLD(Unidirectional Link Detection): is a Cisco-proprietary layer two protocol devised to automatically detect the loss of bidirectional communication on a link monitors a port for bi-directional flow of data. This is done by sending Layer2 UDLD frames identifying the switch port at regular intervals.
The far-end port echos the frames with its own identification added. If UDLD does not receive the echos, the port is either flagged or disabled depending on the configuration. All connected devices must support UDLD in order for the protocol to successfully identify the unidirectional links.
Unidirectional Link Detection (UDLD): The UDLD protocol allows devices connected through media such as fiber-optic or twisted-pair Ethernet to monitor the physical configuration of the cables and detect when a unidirectional link exists. If a unidirectional link is detected, UDLD shuts down the affected port and send out an alert.
UDLD has two modes of operation.
1.Normal mode: Allows the port to operate even after detection of a uni-directional port. A syslog message is generated to alert the administrator.
2. Aggressive mode: Soon after a uni-directional port is detected, an attempt is made to verify the link. If the verification process fails, the link is immediately placed in errdisable state.
By default, UDLD is disabled on a switch. UDLD can be used either on interface basis or globally. To enable UDLD, use the following global configuration command:
Switch(config)# udld {aggressive | enable | message time <seconds>}
Use "aggressive" keyword to enable "aggressive" mode.
Message time <seconds> can be set to any value between 7 seconds and 90 seconds. This is the time period that the switch port echos messages to the neighboring port to find whether the link is operation or not
