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Loop Guard and UDLD

Posted on 2016-08-08
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Last Modified: 2016-08-21
I know that Loop guard is STP (L2) and UDLD is different (L1).
I would like to know if implementing Loop Guard would serve the purpose without implementing UDLD ?  

regarding  Loop guard, I am not sure if it is necessary. I believe in STP by default , if a port does not receive BPDUs ,STP will do its calculation and eventually will have one of the redundant link in blocking mode. So why configure Loop Guard ?

 Thank you
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Question by:jskfan
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LVL 14

Expert Comment

by:SIM50
ID: 41747286
Loop guard doesn't protect against mis-wiring where UDLD does.
UDLD doesn't protect against problems caused by the software where loop guard does.
Overall, both, loop guard and UDLD are pretty close and if you want to implement only one and not the other, it is suggested to go with UDLD. Software glitches are more rare than mis-wiring.

Lets say a port can't receive but can still send data due breaking of one strand of fiber. Can't receive data, can't see BPDUs. It causes the alternate port to transition to forwarding. Now you have one way loop. Configuring loop guard/UDLD helps to prevent this from happening.
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Author Comment

by:jskfan
ID: 41749694
I would say probably UDLD  has capability of catching up the issue before STP knows about it..in other words it is a matter of timing .

In real environment, do Network Engineer configure Loop Guard in every blocked port in the Network ? knowing that Topology can change as well as the ports that are Blocking.
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Expert Comment

by:SIM50
ID: 41750323
Well, once you have a proper network topology with core and aggregation layers, it doesn't change much often. But honestly, in my opinion, L2 networks and STP are becoming obsolete due to the convergence time and wasted links. Plus other factors like seven switch limitation of STP and etc. further decrease the viability in large networks. I attribute this to my lack of seeing the use of Loop Guard in the production environment. For example, in my previous job, all links were routed.
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Author Comment

by:jskfan
ID: 41751379
in my opinion, L2 networks and STP are becoming obsolete
So how do you prevent a Loop from occurring in the Network ?
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Expert Comment

by:SIM50
ID: 41751404
It's all in the design. You design VLANs not to expand passed you aggregation switches. Use routed links. If you need to expand it, like for your vmware servers, you use VXLAN or OTV. You can implement FabricPath and VPCs. There are many options available.
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Author Comment

by:jskfan
ID: 41753231
OK STP is Layer2 , means it does not go beyond L3 device.
VLANs talk to each other through an L3 device (Router or L3 Switch)
If PVST or RPVST is implemented then even if there is an STP Loop, it might impact only hosts in that VLAN, the Loop cannot traverse L3
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Author Comment

by:jskfan
ID: 41760368
You design VLANs not to expand passed you aggregation switches

you mean beyond Core Switches or Distribution Switches ? if you have them both in the design
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Author Comment

by:jskfan
ID: 41760399
Well going back to UDLD and Loop Guard
 if we happen to choose Loop guard only , will it accomplish the same thing as implementing both UDLD and Loop Guard?

Also

if we happen to choose UDLD only , will it accomplish the same thing as implementing both UDLD and Loop Guard?
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Accepted Solution

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SIM50 earned 500 total points
ID: 41760896
UDLD works only on the fiber links. If you use copper than your option is Loop guard.
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Author Closing Comment

by:jskfan
ID: 41764371
Thank you
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