%ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet

[Rojer-bkk]

Hi Expert,

I found error %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet

Now, i disable keepalive and error disappear but i need to know this error cause network loop or software bug?

IOS:12.2(53)SE2

Thanks for all reply

[Chaitra Shree]

Hi,

This message occurs when a keepalive packet is looped back to the port that sent the 
keepalive. STP loops, bad GBIC/fiber, wrong cable being accidently plugged in could be
reasons for the error message to be seen. This is documented in a software bug and the 
recommendation is to disable keepalives. Check for cabling issues,TCNs, mac-mov 
notifications which indicates an STP loop.

Regards,
Chaitra

[Rojer-bkk]

Hi Chaitra,

Thanks for feedback

This error happening for sometimes on either G1/0/23 or 1/0/24 of both swithces

3Com 5500 (Unit 1) port g1/0/15,g1/0/16 -------etherchannel--------Cisco switch1 port G1/0/23,G1/0/24

3Com 5500 (Unit 2) port g1/0/16,g1/0/17 ------etherchannel---------Cisco switch2 port G1/0/23,G1/0/24

As above topology, I can't see the possible to be network loop cause except cabling issue.

[Chaitra Shree]

Hi,

Well, you could concentrate more on eliminating cabling/gbic/port issues and also consider configuring udld port aggresive and stp loopguard feature on the concerned ports to detect any STP failures.

Regards,

Chaitra

[Rojer-bkk]

Hi,

Thanks for recommended. As neighbor switches are 3Com so UDLD couldn't deploy. Enable loopguard is good choice to detect loop.