Udld is a Cisco proprietary L2 protocol.

The switch will send an echo to its neighbor. If the neighbor does not receive a response within a certain period of time, it is deemed that the link with the neighbor has a problem. In normal mode, the link may be regarded as an uncertain state and will not be closed (note that normal mode can only detect misunderstandings of optical fibers, such as Tx and Rx insertion and inversion ). In the aggressive mode, the link is regarded as one-way and closed (the interface is placed in the err-Disable State ).

I. Overview

UdldIsCiscoThe proprietary L2 protocol is used to monitor the physical configuration of the Ethernet link connected by optical fiber or twisted pair wires. When one-way link (only one direction can be transmitted, for example, I can send the data to you, you can also receive the data, but I cannot receive the data you sent me,UdldThis situation can be detected, the corresponding interface is closed, and a warning message is sent. One-way links may cause many problems, especially the Spanning Tree, which may cause loopback. Note: udld can run normally only when both devices on the link are supported.

Ii. udld Working Mode

Udld supports two working modes: Normal Mode (default) and aggressive mode.
Normal Mode: In this mode, the udld can detect one-way links of optical fiber due to port misconnections.
Aggressive Mode: In this mode, the udld can detect one-way links of optical fiber due to port misconnections. In addition, one-way links in fiber and twisted pair links can be detected.

Iii. How udld works

1. Maintain the neighbor Database
The udld periodic type sends a hello packet (also called advertisement or probe) on each active interface ).
After the switch receives the hello packet, it stores this information until the aging time expires. When the switch receives hello again before the aging time expires, it refresh the aging time.
2. Detection and return
The switch will send an echo to its neighbor. If the neighbor does not receive a response within a certain period of time, it is deemed that the link with the neighbor has a problem. In normal mode, the link may be regarded as an uncertain state and will not be closed (note that normal mode can only detect misunderstandings of optical fibers, such as Tx and Rx insertion and inversion ). In the aggressive mode, the link is regarded as one-way and closed (the interface is placed in the err-Disable State ).
The above is just a rough description. There are a lot of content in the Cisco official documentation. If you want to know more, you can check it.

Iv. udld configuration instructions: http://www.openeim.com/

Note:
udld is not enabled globally or any interface by default.
udld does not support ATM interfaces.
make sure that the interface devices at both ends of the link support the udld and must work in the same udld mode.
conf T
udld {aggressive | enable | message time interval} 'enable udld globally, set aggressive to aggressive, and enable to normal, message time: sets the hello interval. The value ranges from 1 to 90. The default value is 15.
int fa0/10
enable udld under the udld port [aggressive] 'interface, without adding aggressive to normal mode.
show udld [interface name] 'to view the udld information.
In udld reset mode, restart all interfaces that are disabled by udld.
some iOS configuration commands may be as follows:
(config) # udld {enable | aggressive}
(config-If) # udld port [aggressive]
(config) # udld message time interval