What is UDLD and why it is required?

Unidirectional Link Detection (UDLD) is a Cisco proprietary layer 2 protocol used to determine the physical status of a link. UDLD helps to prevent forwarding loops and blackholing of traffic by identifying and acting on logical one-way links that would otherwise go undetected.

What is UDLD error?

The UDLD error conditions exist when the switch does not receive the expected information from its UDLD peer. This document describes these UDLD error conditions and how to troubleshoot them: Empty-echo. Transmit-Receive (Tx-Rx) Loop.

How do I reset my UDLD?

After resolving the error condition, we can restore the interface to normal operation either by administratively taking it down and then back up ( shutdown , no shutdown ), or by issuing the global command udld reset to automatically restore all interfaces placed in the error-disabled state by a UDLD failure.

What is UDLD aggressive mode?

With UDLD aggressive mode enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving UDLD packets, UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is disabled.

What causes port to go err disabled?

Causes of Errdisable A port duplex misconfiguration is a common cause of the errors because of failures to negotiate the speed and duplex properly between two directly connected devices (for example, a NIC that connects to a switch). Only half-duplex connections should ever have collisions in a LAN.

Where should Rootguard be enabled?

Root Guard feature can be enabled on switch ports that is connected to other switches that should never become a Root Bridge. For example, a port on the distribution layer switch which is connected to an access layer switch can be Root Guard enabled, because the access layer switch should never become the Root Bridge.

Is UDLD enabled by default?

UDLD is disabled by default. An individual interface configuration overrides the setting of the udld enable global configuration command.

How do I get my port out of err-disabled?

To recover a port that is in an Errdisable state, manual intervention is required, and the administrator must access the switch and configure the specific port with ‘shutdown’ followed by the ‘no shutdown’ command.

What does err-disabled mean?

Errdisable is a feature that automatically disables a port on a Cisco Catalyst switch. When a port is error disabled, it is effectively shut down and no traffic is sent or received on that port. The error disabled feature is supported on most Catalyst switches running the Cisco IOS software.

How do I disable Bpduguard?

To disable BPDU guard, use the no spanning-tree portfast bpduguard default global configuration command. You can override the setting of the no spanning-tree portfast bpduguard default global configuration command by using the spanning-tree bpduguard enable interface configuration command on an STP port.

Why does PortFast not error disable an interface where it receives a spanning tree BPDU?

Though PortFast is enabled the port still participates in STP. If the port happens to be part of topology that could form a loop, the port eventually transitions into STP blocking mode. PortFast is usually configured on an edge port, which means the port should not receive any STP BPDUs.

Why is err-disabled?

The Errdisable error disable feature was designed to inform the administrator when there is a port problem or error. The reasons a catalyst switch can go into Errdisable mode and shutdown a port are many and include: Duplex Mismatch. Loopback Error.