Keepalive loopback cause port err-disable

Unanswered Question
Apr 23rd, 2009
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hi expert here,

i have a problem with the keepalive loopback cause the port error disable.

error logs like below:

%ETHCNTR-3-LOOP_BACK_DETECTED: keepalice packet loop-back detected on

| GigabitEthernet 0/11"

After some investigation, i know that this due to the Keepalive packet source & destination interface having the same address.

And the workaround solution provide by cisco is to disable the keepalive from the related interface. it works fine now.

But i still want to know why cause the Keepalive packet source and destination having the same address.

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Overall Rating: 5 (1 ratings)
Giuseppe Larosa Thu, 04/23/2009 - 23:58
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Hello Frankie,

the keepalives having the same source and dest MAC address = port MAC addresses are correct.

The problem is caused by a Cisco IOS bug where a wrong implementation may treat sometimes the event of receiving back keepalive frames as bad news and sign of a loop.

for keepalive frame format see

[email protected]/msg65904.html')">http:[email protected]/msg65904.html

The bug affects some IOS releases on some switch platforms on fiber based ports.

see this thread^1%40%40.2cc1ebcc/5#selected_message

you may need to copy to browser the text to be able to open the link

you can find this using search box on the top right with key LOOP_BACK_DETECTED

Hope to help


frankie_sky Fri, 04/24/2009 - 00:39
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Hi Giuseppe,

Many thanks for your help. It seems like upgrade the switch IOS version to 12.2(x) will solved the problem permanently.




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