Panic at the Cisco tech, thanks to ancient IOS syntax helper that outsmarted itself

Who, Me? Rejoice, dear reader, for it is Monday – the day to which everyone looks forward. No? Well, the silver lining to this particular cloud is another instalment of Who, Me? – The Reg's weekly tales of techies who thought they were fixing something, but flamed out.

This week's hero is a network tech whom we will Regomize as "Sherlock." Back in the 1990s he was working as an IT manager for a business that had a number of regional hubs, each linked to HQ over a 128K link. Smaller regional offices were linked to their hubs with 64K connections.

Cisco knows that netadmins are a busy lot, and one feature of the command line for its IOS is an autocomplete that means typing some characters of a command followed by the TAB key offers tells the OS to complete whatever syntax you're typing.

Typing sh-TAB can therefore be a shortcut to the command show, thus saving two whole keystrokes and valuable microseconds for greater productivity.

The version Sherlock used back in the day had a similar feature that he discovered could cause problems when driving boxes over a wide area network. So when he typed net sh ip xxx to get settings for a remote box, his network connection disappeared. Attempts to re-establish the link were to no avail – no link existed.

A hasty examination of the Cisco manual revealed the issue: over a network connection, the abbreviation sh corresponds not to show but to shut. Sherlock had destroyed his connection because in the 1990s, when IOS was young, no-one had thought to ensure that the same shortcut didn't mean two entirely different commands in different contexts.

• NetAdmin learns that wooden chocks, unlike swipe cards, open doors when networks can't
• Network engineer chose humiliation over a night on the datacenter floor
• Undergrad thought he had mastered Unix in weeks. Then he discovered rm -rf
• The sad tale of the Alpha massacre

Understanding the issue was one thing – fixing it another. Sherlock managed to find a modem with which he could dial back in to the remote office, and successfully established a connection that way. He still had to re-establish the link to head office though.

He tried the command net open ip xxx, but to no avail. There was still no connection. And the manual was no help this time either.

In desperation he consulted an engineer friend – let's call him "Watson" – and asked if he knew how to detangle this conundrum.

Watson thought for a moment and asked if he had un-shut the network connection. Sherlock thought he had – isn't that what open is for?

No, it seems not. In this ancient edition of IOS, before open would make a connection, users had to reverse the shut condition. So the appropriate command was net no sh ip xxx.

With that done, Sherlock was able to continue his work, with a valuable lesson learned.

Ever had a moment when something you did resulted in something you didn't want to happen, and reading the manual didn't help? Tell us about it in an email to Who, Me? and we may feature your misadventure on some future Monday. ®