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UDLD and vPC Good Mix?

Unanswered Question
Oct 30th, 2014
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If UDLD is configured to detect when a link is still up, but for whatever reason BPDUs have stopped, what use is configuring it on links that are connected to a vPC? Can a VPC configuration still benefit from UDLD?

Note: we've installed a 3850 stack with 2, 10G interfaces configured as a 20G port-channel. 1, 10G connecting to 1 Nexus core 5500 and 1, 10G connecting to the other Nexus core 5500.

We've configured a vPC so both 3850 trunks are forwarding.


Thanks - Pat

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Overall Rating: 5 (4 ratings)
Patrick McHenry Fri, 10/31/2014 - 05:32
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Thanks Guys for the info -.


Is there any danger in enabling UDLD on the Nexus's?


Could anything be negatively effected like FCoE?


I noticed that I couldn't enable UDLD on the Nexus. I have to enable the feature - then, I suppose I will be able to enable it or will it be enabled once I enable the feature?


Thanks, Pat



Reza Sharifi Fri, 10/31/2014 - 05:36
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On nexus you have to enable the feature first than UDLD. UDLD will not be enabled if you just turn on the feature. So, you need both.

Enabling UDLD on Nexus should not be an issue.



Aref Alsouqi Fri, 10/31/2014 - 05:42
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UDLD is a sort of protection as already mentioned, so no danger at all and would not have any negativity as long as you have functional cables.
If you enable UDLD feature, it will be enabled on all the fiber ports globally and disabled on all non-fiber ports, if you want to enable it for non-fiber ports as well, you would still be able to do that by going into the interface config mode.



Aref Alsouqi Thu, 10/30/2014 - 18:21
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Hi Patrick,

UDLD (UniDirectional Link Detection) would not detect any BPDUs, instead it operates at L2 and uses echo messages to detect a hardware (cabling) failure on the link typically for fiber, but it would also work with UTP cables. When UDLD echo messages are not received for the period set in timer the port that not receive those echos would be put in err-disabled state if the UDLD has been enabled in aggressive mode, instead in normal mode the port would be put in undeterminded state. UDLD should be configured on both ends ports, and for fiber ports it would be enabled in config mode, but for non-fiber ports it would be enabled in interface config mode. If an interface goes in err-disabled due the UDLD failure, there is no automatic system to bring it back up unless you use err-disable recovery mechanism, you would still be able to bring it back manually by issuing the command "udld reset" or by shut+no shut the interface. Finally, Cisco recommends to enable both UDLD and Loopguard together, please keep in mind that Loopguard would detect the software issues that may cause STP failure, but not for hardware (cabling) issues, and it has to be enabled on all root and alternate ports, it also has an automatic recovery mechanism to put back a port in normal state once PBDUs are sent back on that port.




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