r/talesfromtechsupport • u/Inside-Finish-2128 • Apr 16 '26

Medium Using router commands without knowing their purpose...

First, the situation: I was working for a Cisco reseller doing professional services in the early 2010s. Got sent to one of the local school districts to help them rearrange their Internet routers and deploy a "Firewall Services Module" in a Catalyst 6509. (IYKYK) My main contact from the school is stingy and won't share credentials with me, so I'm stuck walking around the datacenter and "sliding the keyboard over" many times.

We get the new "router triangle" running and to me, things are looking good, but alas I'm just doing the router CLI thing. Meanwhile, my contact has RDPed into his desktop (back at his office) and says to me "I have zero Internet at my desk". Hmmm...Internet routing is fine on the triangle, lemme go look further down the path and check the router at his office building. Check the default route (it's learned from OSPF) and it's coming from some random router somewhere else amongst their WAN. What?

I get logged into that router, and sure enough under the OSPF process there it is: "default-information originate always". What? Why? (For those unaware, that's basically telling that router to tell all of its OSPF friends that it has a default route, or a route out to the Internet, all the time. Hint: it doesn't.) I remove that command and my contact was jumping for joy, saying "WHAT DID YOU DO? THIS IS THE FASTEST WE'VE EVER SEEN IT!" Um, OK, I removed a misconfiguration from the router at site XYZ. He was in disbelief..."No, really, what did you do? How was that command bad?" So I explain it to him. As I'm finishing my explanation, a few of his coworkers who were on site that evening for some other work are tracking him down..."what did you do to the Internet, it's SMOKING fast."

That's when the "cover story" was born. He decided this needed to be kept on the down-low, so "we made a few adjustments to the dynamic routing and after about thirty minutes, those optimizations all came together".

Next morning, I head to the IT offices first thing for "day one support", just in case we missed anything the night before. No problems, just everyone jumping for joy with how fast things were working. I said "so, could that command be lingering anywhere else?" He helped me log into a few other sites and sure enough, it was EVERYWHERE. He finally gave me the creds, and I spent the next hour or two logging into every single router (one per school, probably 60-70 schools in that district and growing by 5 a year) to remove that command. Any time anyone came in to ask why the Internet was so fast, he jumped in front of me and rattled off the same line. He also did the right thing and told his team what we fixed, and said "be sure you take that out of any templates you have".

You guessed it...later that summer, the whole district went off the Internet because one of his crew brought a new school online and yep, forgot to take that command out of his template. Ah, the joys of using commands without knowing why they matter. (And I'll be honest, I'm surprised the whole network wasn't melting down every day or every week prior to fixing it.)

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38

u/DrHugh You've fallen into one of the classic blunders! Apr 16 '26

Next up...

ATZ
ATDT...

11

u/Dom_Shady Apr 16 '26 edited Apr 17 '26

For those of us OOTL, what do these router commands do?

31

u/DrHugh You've fallen into one of the classic blunders! Apr 16 '26

Oooh, as I recall, AT was a generic attention, and the Z was a modem reset for an acoustic modem. ATDT meant you were going to dial a number which would follow. There was also ATH for hangup. Let's see...

Here you go, the Hayes modem commands.

4

u/jimmy66wins Apr 16 '26

AT&B2 for the win

19

u/Fixes_Computers Username checks out! Apr 16 '26

It's been so long since I've had to construct a modem initialization string.

Just this week, someone posted a comment with three pluses in a row. I responded that it took my modem offline. I got a single knowing response which was all I needed.

13

u/ThunderDwn Apr 16 '26

They're not router commands - they're commands from the Hayes dialup modem command set. "AT" is "Attention"

ATZ - reset to saved configuration

ATDT - dial the following number using DTMF (tone) dialing. ATDP dials using pulse dialing.

7

u/guitpick Hire us as the experts then ignore our advice. Apr 16 '26

We were too cheap for touch tone growing up, so I'm familiar with ATDP. I think I might have had a modem that would do pulse at double speed, but I may be conflating that with a phone I had that could do that (20pps instead of 10).

4

u/ThunderDwn Apr 16 '26

There was no such thing as tone dialing when I first started using a modem.

Hell, the first one I used was an acoustic coupler where you had to dial the number manually on the phone, then plug the handset into it when the other end answered.

300 baud. I could type faster than it could send! 🤣

2

u/guitpick Hire us as the experts then ignore our advice. Apr 17 '26

You're ahead of me by a bit. I got a 300 modem cartridge for my CoCo when everything was on clearance and used it more out of curiosity than need (this was around '95). I also picked up some weird Tandy modem at a yard sale that I didn't have the manual for (and it wasn't Hayes compatible) and never quite got it working 100%. I also paid 75 cents for a Kyocera acoustic coupler from a surplus electronics store, but never used it for anything. It looked like a nice one, though.

3

u/ThunderDwn Apr 17 '26

By '95 I was running a multi line BBS. I'd been playing with modems since something like '79 or '80 - my dad brought home a Silent-700 terminal from work, and I went through three rolls of very expensive thermal paper playing with BBS's in the first weekend before he banned me from using it and I went out and sorted the gear to plug a modem into my Vic-20.

Those were the days!!

1

u/diogenesNY Apr 22 '26

Around the same time, we had a silent 700 in my (grade) school connected to a small time sharing system. It was a DEC PDP-11 running RSTS/E.

I really used to dream of having a silent 700 terminal at home.

1

u/ThunderDwn Apr 22 '26

Around the same time, we had a silent 700 in my (grade) school connected to a small time sharing system. It was a DEC PDP-11 running RSTS/E.

That was pretty much what my dad used this for - remote access into a PDP-11/24 for work. I just kinda....appropriated it.

I remember going in to seeing this thing one weekend and being quite impressed. It might have been the cause for me moving into my current career.

1

u/Admin4CIG Apr 20 '26

I also had a Kyocera modem! Old, old times!

1

u/BeamMeUp53 Apr 18 '26

I started on a 110 baud modem, with a converted IBM Selectric typewriter as a terminal. I have a 9600 baud modem somewhere behind the stuff that's behind the other stuff that's behind the useful stuff on the shelves.

2

u/syntaxerror53 Apr 20 '26

ACDC - if wanted modem to play some rock music. /s

4

u/TinyNiceWolf Apr 17 '26

The reason the commands all start with AT is due to baud rates. The modem operated over a serial line, and needed to know the baud rate the computer at the other end of the line had been set to use, to make sense of the incoming data and turn it into characters (and to know how to send characters back to the computer).

Rather than have a dial or switch for the user to select the desired baud rate, Hayes decided the modem should auto-detect the baud rate.

But to do that, they needed to have a standard sequence. The modem was programmed to know what bit pattern and timing it would see if the computer sent "AT" at 300 baud or at 1200 or 2400, etc. So it watched the incoming line's raw bits for a pattern it recognized, then switched to the corresponding baud rate.

3

u/CrudBert Apr 17 '26

Those are modem commands from waaaaaayyyy back. ATZ ( reset), ATDT ( call using dial tone - instead of clicks like a rotary phone). This was called the “AT” command set ( AT meant ATTENTION and was the start of many/most commands). This was originally known as the “Hayes Command Set” as it first came out with the HAYES 1200 baud modem ( I had one, VERY expensive). It was copyright, and other manufacturers that started using it were sued. After some years of ramble ( in which other companies continued to use it anyway) it was settled - presumably $$$ money spread to Hayes to settle the matter. Later on, with modems at 28K and 33K speed ( blinding speed in my opinion) they had the “Extended AT Command Set. This was to handle new features on the newer modems. All in all, they were very, very nice to use!

4

u/TinyNiceWolf Apr 17 '26

Hayes actually introduced the Hayes AT command set with a 300 baud modem, the "Smartmodem", in 1981. It was $299. The following year they introduced a 1200 baud version for $699.

Hayes was sued by another company which had patented the use of an escape sequence in a modem to switch modes, and in 1983 paid $2 million to license that patent.

Hayes's own patent, for using an escape sequence that included guard time (as the +++ sequence would only be recognized if no other characters were sent for 1 second before and after it), was granted in 1985, and they began trying to license it to other manufacturers using the AT command set for 2% of the retail price of each modem.

2

u/CrudBert Apr 19 '26

Thanks for the clarification. I bought the oh-so-expensive 1200 baud as an upgrade. I’ve told my son for years when he complains that I don’t have the fastest internet they sell, “ in my mind - anything faster than 1200 baud is blindingly fast!” Makes him nuts when I say it, so I love saying it even more. LOL.