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When to use UDLD

Posted on 2016-08-06
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Last Modified: 2016-08-07
UDLD is mentionned in several examples where Fiber Channel connection is used. However I believe it is also used in Copper cable connections when Etherchannel is configured.
For Etherchannel , I thought STP can take care of the loops if any cable goes wrong, no need for UDLD.  
I am not sure if in Fiber Channel we 'll have to configure both  STP and UDLD

Thank you
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Question by:jskfan
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by:Predrag Jovic
ID: 41745925
You can use UDLD on optic cables, copper cables including on etherchannels. UDLD and STP do not exclude each other, you need to configure both (or at least just STP), if you disable STP and enable just UDLD it can lead to network loop. UDLD can be considered a kind of helper for STP (created for specific purpose - detect unidirectional links), addition to make sure that STP will function properly.

You can read for example this article.
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by:jskfan
ID: 41746252
- What do they mean by Unidirectional Link ? if Fiber is used then each link is dual-link (TX/RX)
- If UDLD is implemented and it detects a non-functional cable and turns the port into Errdisabled then what does STP do , as I am assuming UDLD has faster detection than STP ? I know that STP will change the Topology, but what I am looking for is if STP alone can do the job then what UDLD adds as value ?

Thank you
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Expert Comment

by:Predrag Jovic
ID: 41746285
if Fiber is used then each link is dual-link
UDLD (Uni Directional Link Detection) - is all about if only one link gets broken (Tx or Rx) and other link is still active. If only one link is active it can lead to network loop, STP in that case will not function properly.
Unidirectional link is classic example how to create network loop, STP in that case will not detect network loop since it stops receiving BPDUs on inactive link.
what I am looking for is if STP alone can do the job then what UDLD adds as value ?
In the case of unidirectional link STP will not function properly. For that case you need to configure UDLD or LoopGuard.
If UDLD is implemented and it detects a non-functional cable and turns the port into Errdisabled then what does STP do
STP prevents network loop in the case when cable function properly. UDLD detect cable problems and error disables the port if problem is detected.
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Author Comment

by:jskfan
ID: 41746301
Ok in Fiber it makes Sense, since one Cable going to one Port , in fact  behind the scene is 2 cables(RX/TX) going to one port, and STP is based on Port than the physical link itself. UDLD in this case will help STP .


in Copper each cable is going to one Port , if ports are aggregated then it still one cable is going to one port, and STP is capable of detecting the link failure when a port does not receive BPDUs. UDLD in this case , I do not see it where it can help STP
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by:Predrag Jovic
ID: 41746316
in Copper each cable is going to one Port
Let's take for example 100Mb cable link. It uses 2 pair of wires, if one wire is broken (it happens occasionally) only one pair (Tx or Rx) will be active and in that case you have the same effect as in the case of optic cable.
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by:jskfan
ID: 41746334
So UDLD is used in Cables that have RX/TX  going to one port

100 MB is Fastethernet , so it applies to Fastethernet ?
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Predrag Jovic earned 500 total points
ID: 41746434
It applies to both Gig and Fast ethernet links, it is just checking is traffic unidirectional, as long as it is bidirectional everything is OK, otherwise it will error disable port. I used specifcally 100Mb ports for easier understanding.
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