Fast UDLD capability is a feature of network devices that:

• Supports timers in the few-hundred milliseconds range,

• Enables subsecond unidirectional link detection, and

• Facilitates transition between different modes based on configuration.

Fast UniDirectional Link Detection (Fast UDLD) allows detection of unidirectional links in less than a second, with links status messages exchanged every 200 milliseconds. A link’s transition from slow mode to fast mode occurs when both sides of a link have Fast UDLD configured and negotiated successfully to move into fast mode. Conversely, a transition from fast mode to slow mode happens when one of the configured ports has its Fast UDLD configuration removed. Fast UDLD supports a wide range of devices.

UDLD and Fast UDLD supports two modes of operation: normal (the default) and aggressive. In normal mode, UDLD can detect unidirectional links due to misconnected ports on fiber-optic connections. In aggressive mode, UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to misconnected ports on fiber-optic links.

In normal and aggressive modes, UDLD works with the Layer 1 mechanisms to learn the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected ports. When you enable both autonegotiation and UDLD, the Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.

A unidirectional link occurs whenever traffic sent by a local device is received by its neighbor but traffic from the neighbor is not received by the local device.

Normal mode of operation for UDLD

A UDLD normal mode is an operational mode of the Unidirectional Link Detection (UDLD) protocol that:

• Detects unidirectional links caused by fiber misconnection when Layer 1 mechanisms fail,

• Leaves the logical link undetermined if traffic is unidirectional but Layer 1 mechanisms do not detect this condition, and

• Does not take action when autonegotiation detects a physical link issue caused by disconnection of one or more fiber strands.

Aggressive mode of operation for UDLD

UDLD aggressive mode is a protocol mode for Unidirectional Link Detection (UDLD) that:

• Detects unidirectional links using previously defined methods,

• Shuts down affected ports when a unidirectional link is detected, and

• Ensures bidirectional traffic flow on point-to-point links.

In aggressive mode, UDLD not only monitors the health of point-to-point links but also actively disables ports when unidirectional traffic patterns are detected. It is particularly effective in scenarios where device failures occur, such as:

• A port is unable to send or receive traffic,

• One port is down while the other remains up, or

• A fiber strand is disconnected within the cable.

UDLD differs from Layer 1 autonegotiation by operating at a higher layer, ensuring that traffic flows bidirectionally between the correct neighbors.

Example:

• On fiber-optic or twisted-pair links, one of the ports cannot send or receive traffic.

• One of the fiber strands in the cable is disconnected.

In a point-to-point link, UDLD hello packets can be considered as a "heart beat," ensuring the health of the link. A loss of the "heart beat" indicates a failure that requires the link to be disabled unless a bidirectional connection can be reestablished.

UDLD operates by using two methods:

• Neighbor database maintenance

• Event-driven detection and echoing

How UDLD maintains neighbor databases

The process of UDLD neighbor database maintenance ensures that each device stays updated about its neighbors, maintains synchronized caches, and handles exceptional cases like errors or configuration changes.

The key components involved in the process are:

• UDLD-capable neighbors: Devices that exchange UDLD hello packets for neighbor discovery and synchronization.

• Hello packets: Periodic packets sent to exchange device information and keep caches updated.

• Cache entries: Stored information about neighbors with an expiration mechanism (age time).

• Configuration changes: Events such as port status changes, UDLD enable/disable actions, or device resets that trigger cache clearing.

The process ensures accurate and synchronized neighbor database maintenance. However, interfaces receiving multiple device IDs will be placed in an error-disabled state to avoid misconfiguration.

Event-driven detection and echoing

UDLD relies on echoing as its detection operation. Whenever a UDLD device learns about a new neighbor or receives a resynchronization request from an out-of-sync neighbor, it restarts the detection window on its side of the connection and sends echo messages in reply. Because this behavior is the same on all UDLD neighbors, the sender of the echoes expects to receive an echo in reply.

If the detection window ends and no valid reply message are received, the link might shut down, depending on the UDLD mode. When UDLD is in normal mode, the link might be considered undetermined and might not be shut down. When UDLD is in aggressive mode, the link is considered unidirectional, and the port is disabled.

UDLD reset options

If an interface becomes disabled by UDLD, you can use one of the following options to reset UDLD:

• The udld reset interface command.

• The shutdown interface configuration command followed by the no shutdown interface configuration command restarts the disabled port.

• The no udld { aggressive | enable} global configuration command followed by the udld { aggressive | enable} global configuration command reenables the disabled ports.

• The no udld port interface configuration command followed by the udld port [ aggressive] interface configuration command reenables the disabled fiber-optic port.

• The errdisable recovery cause udld global configuration command enables the timer to automatically recover from the UDLD error-disabled state, and the errdisable recovery interval interval global configuration command specifies the time to recover from the UDLD error-disabled state.

The udld port disable command disables UDLD on fiber-optic LAN ports.

Note:

This command is only supported on fiber-optic LAN ports.

Default UDLD configuration

Default UniDirectional Link Detection (UDLD) configuration governs network communication integrity by detecting unidirectional links.

The default settings for UDLD configuration are as follows:

This set of configurations ensures that UDLD operates under default settings until explicitly enabled or modified to suit specific network environments.