re-enabling port after error

[nestor-tytla]

Hello,

 I have a port down on a Catalyst 6513 that went down because of the following error is log

Nov 6 13:16:22.127 EST: %RADIUS-4-RADIUS_DEAD: RADIUS server 10.195.16.100:1645,1646 is not responding.
Nov 6 13:16:22.127 EST: %RADIUS-4-RADIUS_ALIVE: RADIUS server 10.195.16.100:1645,1646 has returned.
Nov 6 15:54:01.955 EST: %RADIUS-4-RADIUS_DEAD: RADIUS server 10.195.16.100:1645,1646 is not responding.
Nov 6 15:54:01.955 EST: %RADIUS-4-RADIUS_ALIVE: RADIUS server 10.195.16.100:1645,1646 has returned.
Nov 6 16:40:17.488 EST: %UDLD-SP-4-UDLD_PORT_DISABLED: UDLD disabled interface Gi9/2, aggressive mode failure detected
Nov 6 16:40:17.488 EST: %PM-SP-4-ERR_DISABLE: udld error detected on Gi9/2, putting Gi9/2 in err-disable state

Where can I find commands to diagnose this port or to restore it to service ?

[Mark Elsen]

  - @nestor-tytla    In this case the UDLD errors are a diagnostic pointing to a fiber path problem , being no longer
                             bi-directional.

  M.

[Mark Elsen]

  - @nestor-tytla       Informational ; ref :  https://www.networkacademy.io/ccna/spanning-tree/udld

>...

UDLD Normal vs. Aggressive Mode

UDLD (Unidirectional Link Detection) can be configured to operate in two different modes: normal and aggressive.

• In normal mode, UDLD just monitors the link. If it doesn't hear echoed probes from the remote side for a while, it marks the link as "undetermined" but doesn’t shut it down. This can be useful on long-haul fiber links where one-way communication is still better than no communication. However, this mode is not appropriate for links that are part of the STP domain because the port stays up and continues to follow the Spanning Tree Protocol (STP) rules (which could lead to a loop).
• In aggressive mode, things work a bit differently. If UDLD stops receiving probes but the link still looks up, it tries several times to re-check the connection. If it can’t confirm the link is working both ways, it shuts down the port by putting it into an err-disable state. This immediately prevents all problems associated with a one-way link.

In summary, normal mode detects unidirectional links and logs an error without shutting down the port. Aggressive mode puts the link in err-disable mode when it detects a problem. Ninety-nine percent of the time, real-world environments run the protocol in aggressive mode.

Configuring UDLD

UDLD can be enabled globally (for fiber ports only) or per interface (for any media). 

We enable the protocol globally on a Cisco device using the following command in global config mode:

Switch(config)# udld enable

By default, the global command enables unidirectional detection only for fiber ports. It does not activate the protocol on copper Ethernet ports unless you enable it manually on the interface.

We enable the unidirectional detection on an interface level using the following command:

Switch(config)# interface Ethernet0/0
Switch(config-if)# udld port aggressive

This command overrides the global configuration and enables the protocol on the port irrespective of the media type.

[pieterh]

of course first step is to investigate and repair the reason for the unidirectional link.
-> measure the fiber's charcteristics,
- using a simple light source, in forward and return path
- or better use a real fiber measuring tool.

but (if unidirectional link is resolved) you can restore service by either
- shutdown / no shutdown on the interface (errdisable is cleared by this sequence)
- enable "errrdisable recovery udld" in your configuration (errdisable is automatically cleared after corresponding timer expired)

if you use above commands, and the link is still unidirectional, then the errdisable state will return.

[nestor-tytla]

Unfortunately unable to resolve this issue as it is not a problem with this router or fiber path. The router that these ports connect to is now a dead Cisco 6500 for which there is no longer support. I will be opening a new thread with questions there for how to recover That is possible.

Thanks You all