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TAC Tip: Solutions for the December TS Newsletter TAC Mystery

The December Technical Services Newsletter featured the very first TAC Mystery; an article that invited newsletter readers to help solve a challenging TAC case that was reported over 80 times in 2010.

Here is an excerpt from the mystery:

A few years back, we discovered a bug in the async drivers used in Cisco IOS.

In a nutshell, when one of the modem control or flow control input leads changed state, the TTY line would be put on an internal queue for later processing, and this could happen multiple times. If it happened rapidly enough, the TTY could use up all of the buffer elements allocated in the router - resulting in a SYS-3-NOELEMENT error message, among other things.

All entries were reviewed by a TAC engineer.  The one that was judged to contain the most helpful example, with useful root cause details, was submitted by Daniel Lob.  Daniel wrote, "In my case it was an external modem with a defective power source."

Other people provided clues to what may be root causes:

  • Peter Welcher, who was also the first person to respond, talked about long, unconnected cables
  • Jay Masuda talked about twisted pair being used and data leads interacting with control leads
  • Frans Brinkman suggested it might be sensitive receivers
  • Gene Star postulated on all three of the causes listed above

These three possible contributing factors warrant more investigation.

We also appreciated entries from Joe Koch and Lee Smith.

Thanks to everyone who sent in entries!  Each participant mentioned in this article will receive a Cisco shirt. Be looking for the next TAC Mystery, as well as other TAC tips, in the TS Newsletter.

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