[c-nsp] %ETHCNTR-3-LOOP_BACK_DETECTED on cisco 3550

[Michael K. Smith]

Hello All:

On 8/31/06 9:33 AM, "Kevin.X.White at corusgroup.com"
<Kevin.X.White at corusgroup.com> wrote:

> 
> 
>> I saw that Brett on March 31 had this same problem. I was wondering
>> what he did to resolve this issue. My 3550 has been running for 2 years
>> and just started this problem.
> 
>> Thanks Keith
> 
> 
> 
>> strange problem on 3550G and 2950G switches: 3550 is attached via MM SX
>> GBIC to a 2950G (same GBIC). equipment burned in for a week, and has
>> slowly started exhibiting this problem on the fiber link between these
>> two switches: Loop-back detected o:18.751 MST:
>> %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on
>> GigabitEthernet0/10. The port is forced to linkdown. 000068: *Feb 28
>> 18:36:20.751 MST: %LINK-5-CHANGED: Interface GigabitEthernet0/10,
>> changed state to administ ratively down 000069: *Feb 28 18:36:21.751
>> MST: %LINEPROTO-5-UPDOWN: Line protocol on Interface
>> GigabitEthernet0/10, chan ged state to down the error message on
>> cisco.com states: Error Message ETHCNTR-3-LOOP_BACK_DETECTED:, Loop-back
>> detected on [chars]. The port is forced to linkdown. Explanation This
>> message means that the loop-back condition might be caused by a balun
>> cable being accidentally plugged into the port. [chars] is the port.
>> Recommended Action Check the cables. If a balun cable is connected and
>> the loopback condition is desired, no action is required. Otherwise,
>> connect the correct cable, and bring the port up by entering the no
>> shutdown interface configuration command. given that this is a fiber
> c>onnection (tried both 10 and 15ft. fiber patch cables), obviously the
>> cisco explanation doesn't fly. has anyone seen a similar problem? i've
>> swapped GBICs, fiber patch cables, moved the GBICs to different ports on
>> *both* switches, but the error message above *always* shows up on the
>> 3550, and the 3550 shuts the link down. 3550 is running 12.1(12c) EA1
>> 2950G is running 12.1(11) EA1 only thing i can think is a software bug
>> on the 3550 (other 2950 attached with same GBICs and fiber patches does
>> not exhibit this problem).
> 
>  I've had the same problem, the misleading thing is that the not all 2950s
> will shutdown under these conditions but 3550s do
>  In our case a modem the other end of a load of 'dumb' L2 only DEC hardware
> had the loopback button in the only hardware that cared was a 3550 at the
> edge!!
>  This shut down a section of our production network for HOURS.
>  When this as reproduced in the lab with a couple of 2960s only 1 unit
> would exhibit the warning even though the same IOS & Zilch config!!
> 

Check out:

http://www.ciscotaccc.com/kaidara-advisor/lanswitching/showcase?case=K588581
48

Basically, the solution is to disable keepalives on the affected interfaces.
We had this happen and disabled keepalives which didn't have any adverse
effects on the network for over a year.

Mike