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Unanswered Question
Oct 18th, 2005
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one of my 3550 catalyst switch Port get always the message "LOOP_BACK_DETECTED: Keepalive packet loop-back d

etected on GigabitEthernet0/11". This Port goes then in the err-disable state. When I take the syntax "no errdisable detect cause loopback" then I get on an another switch the message "LOOP_BA..." too. What can I do?

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spremkumar Tue, 10/18/2005 - 01:58
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In most cases its the faulty cable or the bad patching done which triggers this error log in the catalyst switch and also put the port in errordisable mode.

do revert what kinda equipment is getting connected on the other end of this switchport.


amit-singh Wed, 10/19/2005 - 03:51
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On Switches there is not much use of keepalives . Keepalives are just used as hellos between the 2 switches.They are not used to check the link reliability or error detection on the link as used in case of routers. You can safely remove the Keepalives from your switch interface and enabled UDLD inturn to check the uni-directional link problem.

UDLD is designed to work specially on GIG interfaces.

To remove the keeplaives:

conf t

int gig 0/11

no keepalive

To enable the UDLD:



-amit singh

mnewnam01 Wed, 10/19/2005 - 07:30
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It has been my understanding that you typically use keepalives for copper ports and UDLD for fiber ports. Is this not correct?

szahid Wed, 10/19/2005 - 09:39
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that is right. It is not advisable to remove keepalives from the copper ports.Loopback detection through keepalives was a mechanism that was put in place to detect loops in the network

caused by Type1a or tyep2 copper cabling. While it is a good mechansim for those special

situations , there seems to be no need for having it enabled on the Gig ports where you

can use features like UDLD to detect a fiber loop or unidirectional fiber.

Salman Z.


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