Fiber link fails to pass traffic

Unanswered Question
Sep 29th, 2007


I am a Systems Engineer working with a Cisco Channel Partner.

A client of ours has a fiber backbone between two buildings on their campus.

The fiber terminates on a Catalyst 6509 (at the data centre) and a Catalyst 3750 at the other end. Users at the far-end connect to servers located at the data center over this fiber.

This connection has been working fine for the past 2 years, until a recent problem came up.

For two weeks now, the usual occurrence is for the fiber link to stop passing traffic while working until the 3750 is rebooted or the fiber connectors removed from the SFP port and replugged.

Meanwhile, the interface and line protocol remain UP on both switches during this abnormal behaviour. There are also no log messages occurring. Everything look fine on both switches. They just wouldnt pass traffic. The problem occurs about twice each day.

It's difficult diagnosing the problem now. I need help pls.



I have this problem too.
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balajitvk Sat, 09/29/2007 - 10:56


I hope this link is Layer2 links with trunk enabled. If so, see whether any spanning tree blockings in that time.....


felixnkansah Sat, 09/29/2007 - 11:02


The fact is, its a Layer 3 link (with IP addresses assigned to both ends of the connection).

During this period of abnormal behaviour, it is not possible to even PING the other ends however.


balajitvk Sat, 09/29/2007 - 11:15


Which interface IP is not pingable 3750 or 6500??? Whether able to login thro. console on that time or not??? Sometimes spikes in the processor utiliztion may leave this effect for shorter duration.


felixnkansah Sat, 09/29/2007 - 11:27


I am not able to ping any of the interfaces from either switches. I mean there is no connectivity whatsoever from one end of the link to the other, even though line protocol remains up during this period.

I am able to console-in to the switches.

The fact is, when the problem occurs, the link never comes back on until we unplug and replug the connectors to the 3750 or 6509, or restart the 3750.


balajitvk Sat, 09/29/2007 - 11:44


Could u paste the configuration on the both side switch ports???


paul.matthews Sat, 09/29/2007 - 12:12

Are the interface RX counters incrementing during the problem? What do the buffers look like? I presume CDP goes, but one way or both?

What versions of IOS?


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