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Rojer-bkk
Beginner

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

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

4 REPLIES 4
Chaitra Shree
Cisco Employee

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


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.

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

Hi,

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