Giuseppe Larosa Mon, 03/16/2009 - 06:29
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Hello Moamen,

this error counter increments when frames that are correct at OSI layer 2 carry inside a protocol type that is not known or not running on the fas0/0 interface.


IS-IS packets are not ip packets we use it for routing, access layer switches count is-is hellos as unknown because they are not running IS-IS.

There are other possible reasons : ipv6 packets or layer 2 signalling packets like VTP or STP and others

if they increment regurarly over time like 1 one every 10 seconds or some every few seconds the reason is a not understood protocol.

if the number is not great you should be fine (no issues for your router)

Hope to help


moamen.elhefnawy Mon, 03/16/2009 - 06:53
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Thanks fr your help, just do you have any document on Cisco support that , as our customer need such document becuse he think it is a problem .

Thanks & BR


eborcher Mon, 03/16/2009 - 13:12
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Can you provide us with the following output:

(1) show interface fastethernet 0/0 counter

(2) show interface fastethernet 0/0

(3) show version


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