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Only Line protocol goes up and down

hello everybodey,

in my opinion I got a strange problem on my 3750 Switch:

2 Switches connected with each other and on only one switch I can see loggs about line protocol going up and down every 5 minutes.

on the other Switch I can't see any loggs about interface going up and down.

the switch is an 3750-12S with a 100/1000BaseSX SFP on it.

I'm a little bit lost here?

Thanks in advance



Re: Only Line protocol goes up and down

What does each of the transceivers show when issued the following command:

show interfaces interface-id transceiver detail

I am suspecting a possible dirty connector on one fiber that may be resulting in a low receive signal at one end of the circuit. If you swapped (switched) the fibers on both ends, does the problem continue or move to along with the fiber?


Re: Only Line protocol goes up and down

thanks baileyshbr for the fast response. Unfortunatly i can't execute the command show interfaces GiX/X/X transceiver detail.

In the meanwhile I changed the SFP incl the fibre to another free port and its stable now.

Any thoughts? I'm just curious about this problem. What does it mean if the log only show line protocol up / down but not the link itself.


Re: Only Line protocol goes up and down

Information pieced together from the web.

The Link LED indicates that the port is properly  connected to a  powered-on device. this LED is on when the Link Integrity Test (LIT) passes  and off when the link integrity test fails.

The LINK LED is controlled by the Link Integrity Test, which determines the health of the link. Immediately after powerup, the physical medium attachment (PMA)  transmits a normal link pulse (NLP) to tell the NIC at the other end of  the link that this NIC wants to establish an active link connection:

If the NIC at the other end of the link  is also powered up, it responds with its own NLP.

If the NIC at the other end of the link  is not powered up, this NIC continues sending an NLP about once every  16 ms until it receives a response.

The link is activated only after both NICs are capable of exchanging  valid NLPs.

So a cable can be       just good enough to connect at the physical layer, but it corrupts  packets as a       result of subtle damage to the wiring or connectors.  In your case, a dirty or damaged connector or jumper may have resulted in high data errors.  The errors may not have been severe enough to stop the LIT to fail, but the Ethernet frames were being corrupted enough to cause the protocol to shut it down.   

Hope this helps!