UDLD Modes

[CiscoBrownBelt]

Cisco docs are a bit confusing on this. So if UDLD normal mode is used, does the port/link remain up even when a unidirectional link failure is detected? 

[balaji.bandi · Accepted solution]

UDLD can operate in two modes: normal and aggressive.

In normal mode, if the link state of the port was determined to be bi-directional and the UDLD information times out, no action is taken by UDLD. The port state for UDLD is marked as undetermined. The port behaves according to its STP state.

In aggressive mode, if the link state of the port is determined to be bi-directional and the UDLD information times out while the link on the port is still up, UDLD tries to re-establish the state of the port. If not successful, the port is put into the errdisable state.

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10591-77.html

[CiscoBrownBelt]

Yes thanks but what if unidirectional link failure is detected in normal mode?

[Reza Sharifi]

Balaji already posted the answer to your question. You can also set up alerts, so you get text, email or both (depending on the application capabilities). This way if there is something wrong with the link in normal mode, you know about it and can fix it before it breaks.

HTH

[luis_cordova]

Hi @CiscoBrownBelt,

Adding to what is indicated by @balaji.bandi, the following paragraph gives more clarity to your query:

UDLD information can age out due to the high error rate on the port caused by some physical issue or duplex mismatch. Such packet drop does not mean that the link is unidirectional and UDLD in normal mode will not disable such link.

Regards