Layer 2 Switching Software Configuration Guide for Cisco IE 2000U and Connected Grid Switches

Modes of Operation

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.

In normal mode, UDLD detects a unidirectional link when fiber strands in a fiber-optic port are misconnected and the Layer 1 mechanisms do not detect this misconnection. If the ports are connected correctly but the traffic is one way, UDLD does not detect the unidirectional link because the Layer 1 mechanism, which is supposed to detect this condition, does not do so. In this case, the logical link is considered undetermined, and UDLD does not disable the port.

When UDLD is in normal mode, if one of the fiber strands in a pair is disconnected, as long as autonegotiation is active, the link does not stay up because the Layer 1 mechanisms detects a physical problem with the link. In this case, UDLD does not take any action and the logical link is considered undetermined.

In aggressive mode, UDLD detects a unidirectional link by using the previous detection methods. UDLD in aggressive mode can also detect a unidirectional link on a point-to-point link on which no failure between the two devices is allowed. It can also detect a unidirectional link when one of these problems exists:

• On fiber-optic or twisted-pair links, one of the ports cannot send or receive traffic.
• On fiber-optic or twisted-pair links, one of the ports is down while the other is up.
• One of the fiber strands in the cable is disconnected.

In these cases, UDLD disables the affected port.

In a point-to-point link, UDLD hello packets can be considered as a heart beat whose presence guarantees the health of the link. Conversely, the loss of the heart beat means that the link must be shut down if it is not possible to re-establish a bidirectional link.

If both fiber strands in a cable are working normally from a Layer 1 perspective, UDLD in aggressive mode detects whether those fiber strands are connected correctly and whether traffic is flowing bidirectionally between the correct neighbors. This check cannot be performed by autonegotiation because autonegotiation operates at Layer 1.

Methods to Detect Unidirectional Links

UDLD operates by using two mechanisms:

• Neighbor database maintenance

UDLD learns about other UDLD-capable neighbors by periodically sending a hello packet (also called an advertisement or probe) on every active port to keep each device informed about its neighbors.

When the switch receives a hello message, it caches the information until the age time (hold time or time-to-live) expires. If the switch receives a new hello message before an older cache entry ages, the switch replaces the older entry with the new one.

Whenever a port is disabled and UDLD is running, whenever UDLD is disabled on a port, or whenever the switch is reset, UDLD clears all existing cache entries for the ports affected by the configuration change. UDLD sends at least one message to inform the neighbors to flush the part of their caches affected by the status change. The message is intended to keep the caches synchronized.

• Event-driven detection and echoing

UDLD relies on echoing as its detection mechanism. 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 is 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.

If UDLD in normal mode is in the advertisement or in the detection phase and all the neighbor cache entries are aged out, UDLD restarts the link-up sequence to resynchronize with any potentially out-of-sync neighbors.

If you enable aggressive mode when all the neighbors of a port have aged out either in the advertisement or in the detection phase, UDLD restarts the link-up sequence to resynchronize with any potentially out-of-sync neighbor. UDLD shuts down the port if, after the fast train of messages, the link state is still undetermined.

Figure 15-1 shows an example of a unidirectional link condition.

Figure 15-1 UDLD Detection of a Unidirectional Link

Enabling UDLD Globally

Follow this procedure to enable UDLD in the aggressive or normal mode and to set the configurable message timer on all fiber-optic ports on the switch.

BEFORE YOU BEGIN

Review the “Information About UDLD” section and “Guidelines and Limitations” section.

DETAILED STEPS

To disable UDLD globally, use the no udld enable global configuration command to disable normal mode UDLD on all fiber-optic ports. Use the no udld aggressive global configuration command to disable aggressive mode UDLD on all fiber-optic ports.

EXAMPLE

The following example shows how to enable UDLD in normal mode on all fiber interfaces:

Enabling UDLD on an Interface

Follow this procedure either to enable UDLD in the aggressive or normal mode or to disable UDLD on a port.

BEFORE YOU BEGIN

Review the “Information About UDLD” section and “Guidelines and Limitations” section.

DETAILED STEPS

EXAMPLE

The following example shows how to cause a port interface to enable UDLD in the aggressive mode regardless of the current global udld (aggressive or enable) setting:

Resetting an Interface Disabled by UDLD

Follow this procedure to reset all ports disabled by UDLD.

BEFORE YOU BEGIN

You can also bring up the port by using these commands:

• 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 re-enables the disabled ports.
• The no udld port interface configuration command followed by the udld port [ aggressive ] interface configuration command re-enables 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.

DETAILED STEPS

EXAMPLE

This example shows how to reset all the ports that are error disabled by UDLD: