UDLD Question

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Feb 3rd, 2009
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Dear all,

<br />

<br />Like I understand udld, it will give me a syslog output or shut down the port depends on the mode (normal(aggressive)

<br />

<br />Now I configured it in the lab and "damaged" one of the both fibers. The Link goes down but UDLD was not the reason like i saw.

<br />I'm wrong with my thinking, anyone an idea how i can check the udld in the lab?

<br />

<br />

<br />tested with 3750 and SFP mulimode.

<br />

<br />thanks!

<br />Best regards

<br />Sebastian

Correct Answer by Yudong Wu about 8 years 3 months ago

The way you test it is correct. If you disconnect one strand of fiber, it should cause a unidirectional link.


Before you create unidirectional link, can you use "show udld gig 1/0/1" on both sides to see if they have learned the neighbor.

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Roberto Salazar Tue, 02/03/2009 - 09:27
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UDLD is a Layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables to monitor the physical configuration of the cables and detect when a unidirectional link exists. All connected devices must support UDLD for the protocol to successfully identify and disable unidirectional links. When UDLD detects a unidirectional link, it administratively shuts down the affected port and alerts you.


When UDLD is in normal mode, if one of the fiber strands in a pair is disconnected and autonegotiation is active, the link does not stay up because the Layer 1 mechanisms did not detect 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 interfaces cannot send or receive traffic.


•On fiber-optic or twisted-pair links, one of the interfaces is down while the other is up.


•One of the fiber strands in the cable is disconnected.


In these cases, UDLD shuts down the affected interface.


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 determines 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.

Sebastian Helmer Wed, 02/04/2009 - 08:26
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Thanks very much, I already read this in the config guide ;-)..

But I'm not really sure if i need udld really.

I disabeld all link detection mechanism (flapping etc..) and udld is not working when I damage one fiber strands. I know this is a layer 1 mechanism.


So do you have an idea how i can create a situation where udld will work? I can't really imagine a situation where i need it.maybe I have to less experience.......;-)

Yudong Wu Wed, 02/04/2009 - 10:44
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UDLD is for preventing spt loop in case of unidirection link. You can also use loop gard. The following link is their comparsion.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094640.shtml#loop_guard_vs_uld


If the switch is only connected to one other switch, in other word, there is no physical loop in your topology, it's Ok for you not to enable it.


Did you enable UDLD under the port when you did the testing? Check it with "show udld int x/y" command.

Sebastian Helmer Wed, 02/04/2009 - 12:39
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Thanks for your replay.

I enabled agresssive mode under the interface gig 1/0/1 and on 1/0/1 on the other switch.

And I understand how udld work, etc. "I think I do"

But i want create a situation where udld works and err-dissabled the port, this is only for me in a lab to see it ...

But i think i can't create a situation where one fiber is sending but not receiving packets, because when I damaged one of the both, the port goes down.





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Yudong Wu Wed, 02/04/2009 - 12:48
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The way you test it is correct. If you disconnect one strand of fiber, it should cause a unidirectional link.


Before you create unidirectional link, can you use "show udld gig 1/0/1" on both sides to see if they have learned the neighbor.

Sebastian Helmer Wed, 02/04/2009 - 12:51
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Ok I will check it once again in the lab tomorrow..

Thanks all for the discussion...


Yudong Wu Wed, 02/04/2009 - 12:58
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Personaly, I did not test UDLD like what you did. But I think it should work based on its mechanism. By the way, I believe you have turned off auto-negociate per your previous post.

Looking forwarding to your test result.

Sebastian Helmer Thu, 02/05/2009 - 12:07
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Hey Kevin,


just for you information. I did it today and disable all features i know. link error detections, use switch noneg...! I damaged ones again on of the both fibers, the port goes down nothing more ;)..strange, but I think I can't create a situation where udld will work. but I think this feature is helpful and I will use it anymore. It was just for me to play ;-)..

rduke Thu, 02/05/2009 - 14:10
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FYI, I learned the hard way about UDLD. Where I used to work, we had a lot of 62.5 micron multimode fiber. Some of it was not so good for gigabit, but we were able to make it work using the mode conditioning patch cables; however, a few links were still having errors. Unfortunately, we had rapid spanning tree on some of these links and the errors were unidirectional. This triggered loops and we literally had to run all over a plant to get to the switches and shut down the errored links with laptops using the console ports. Since then I have used UDLD.


Randy

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