[nsp] error on ethernet controller cisco 3550

brett watson brett at the-watsons.org
Mon Mar 31 18:24:04 EST 2003

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 connection (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 


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