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

[Kevin.X.White at corusgroup.com]

 >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!!

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