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Rizwan Khan
Level 1
Level 1

Hello everyone. Can some one help me out in understanding UDLD feature on Cisco switches.tried googling it but could not understand.
Thanks in advance.

Sent from Cisco Technical Support Android App

17 Replies 17

It has to be said, though, that the normal mode was always somewhat of a mystery to me. There are conflicting descriptions being circulated about it. 

No wonder I've never heard of "normal mode".  LOL

Unfortunately for me, I've never had the chance to try out Fast UDLD.  But it's very beneficial when you're dealing with 10 Gbps and up. 

Hello Peter,

I had some problems too with those conflicting descriptions:

Meanwhile I found some additional information in BRKRST-2333 - Network Failure Detection:

I have to say that's still not totally clear for me (especially the formulas for the detection time) but as far as I understand it, faulty link detection and time period refer to the UDLD Hellos.


1) Hellos are send on a local link and

2) Hellos are received on that link from a neighbor and

3) the own port-ID is not seen in the received Hello

the link is considered as faulty and will be error disabled.

Best regards


Here is another link, which said normal and aggressive mode can both lead to error disable mode.

and the recommand is normal mode.

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