IOS: Console, Terminal, Monitor, VTY — What Is What ?

IOS speak– Monitor, Terminal, VTY or Console ?

When you are con­nect­ing to your Cisco IOS router, we use a num­ber of dif­fer­ent terms for ALMOST the same thing. Lets cover this syn­tax so that you can speak the same lan­guage as me. That way, I will like you bet­ter and we won’t make mistakes.

Syntax

Herewith is the offi­cial Ethereal Mind defin­i­tion for each term. Do not use any other meaning.

Console

Console as mean­ing serial con­nec­tion when you con­nect the con­sole cable to the device, and the serial port to your laptop /​ com­puter.

Monitor

Monitor is a tel­net /​ SSH ses­sion, but can some­times be known as the Terminal¹. In rather spe­cial cir­cum­stances, the Monitor can also be known as the “VTY”.

Terminal

Terminal is USUALLY a more broad term for BOTH CONSOLE and MONITOR.

VTY

A VTY is term used by Cisco to describe a single ter­minal (whereas Terminal is more of a verb or gen­eral action term).

You say Terminal Program not Console Program

Most people also call the soft­ware on their desktop, their “Terminal Program” or sim­ilar. And they typ­ic­ally say “I will ter­minal into the router”, or pos­sibly “I will tel­net to the router” or even “I will SSH into the router”.

And when you are con­nect­ing the the blue serial cable, you typ­ic­ally say “I am con­sol­ing into the box”.

That’s what we say, and that’s what you should say. Then we all mean the same thing.

Footnote: Apocryphal Note on VTY for younger people

The fol­low­ing is con­jec­ture on my part, I have no actual proof that this is true but it seems highly likely.

So what’s vty then ? A tele­type! Are you kidding?

vty is short for Virtual TeletYpe. Teletype refers to the days when com­puters were pro­grammed by char­ac­ter based print­ers, lit­er­ally, a key­board attached to a printer. When you pressed a key, the char­ac­ter was prin­ted on paper. Screen to dis­play char­ac­ters were inven­ted much later. That’s how early com­puters go input. Go and look at http://​en​.wiki​pe​dia​.org/​w​i​k​i/Teletype for full details.

So when Cisco IOS was first star­ted out, it was based on a OS that still regarded these keyboard/​printers as the only way to I/​O the sys­tem. I guess Cisco never got around to chan­ging it.²

Oh yeah, and the very early mod­els where called LINE PRINTERS. That’s because you had to type an entire line and press enter before the printer would print it. If you made a mis­take you had to delete the entire line, type it again and hope it was right.³. That’s why Cisco con­tin­ues to call a ter­minal ses­sion a line.

A tele­type (as stolen from http://​www​.pdp8​.net/​a​s​r​3​3​/​a​sr33.shtml)

For the record, very early ver­sions of the Cisco CLI used the “line printer” idea to input con­fig­ur­a­tion com­mands. Terry Slattery talks about it here. Here is an excerpt:

The ori­ginal Cisco router didn’t even have a CLI. According to Kirk Lougheed, one of the founders of Cisco, it was only inten­ded to have its con­fig­ur­a­tion loaded via TFTP. He told me that he needed the abil­ity to change the con­fig­ur­a­tion at a trade show, so he added a quick hack to allow him to type the con­fig­ur­a­tion into a buf­fer, which was passed to the func­tion that parsed the TFTP file. The end of the input was indic­ated with CTRL-​​Z. You entered all the com­mands and when you pressed CTRL-​​Z, the file was parsed and any errors were dis­played. It wasn’t great to have a bunch of typ­ing be wasted when an error occurred.

Some time later, the CLI was changed to cause lines to be executed as soon as they were entered instead of after the CTRL-​​Z was input. Greg Satz, who told me of this change, was pleased to note that I had just barely noticed the change. I dis­tinctly recall think­ing that some­thing changed, but hadn’t been able to put my fin­ger on exactly what. The change repor­ted errors as soon as you entered them, not after the entire buf­fer had been typed, so it was a good change. This change would have happened some­time before late 1990. There was still no com­mand his­tory, inter­act­ive help, or com­mand edit­ing capability

Which vty am I con­nec­ted to ?

Typically, an IOS router has five VTY ses­sions by default although you can con­fig­ure more. This means five con­cur­rent tel­net /​ SSH ses­sion. If you con­fig­ure more, you can (depend­ing on IOS ver­sion) have more. For example, IOS SX code (switches) has fif­teen by default.

line vty 0 4
exec-timeout 0 0
privilege level 15
password 7 094F471A1A0A
no login
transport input telnet
line vty 5 15
no login
!

show ter­minal

You can tell which line printer you are using by using the “show ter­minal” com­mand and the line num­ber shows you which “line” you are con­nec­ted to. That’s why ter­minal ses­sions still talk about width and length: because you needed to know what sort of paper you had loaded into tele­type and the ter­minal had to have set­tings for the paper size.

s1#sh terminal
Line 2, Location: "", Type: "LINUX"
Length: 62 lines, Width: 161 columns
Baud rate (TX/RX) is 9600/9600
Status: PSI Enabled, Ready, Active, No Exit Banner, Ctrl-c Enabled
Automore On
Capabilities: none

Wrap Up

Seriously, it’s the little things that can make a dif­fer­ence. If every­one is call­ing some­thing a par­tic­u­lar name, it doesn’t really mat­ter what you think. Using dif­fer­ent terms can cause errors and mis­takes. And while it’s unfair that older or more exper­i­enced engin­eers con­tinue to use weird ter­min­o­logy, chan­ging it can often cause prob­lems. Get over it.

Just like in Certification Exams, some­times some­thing “just is that way”. Don’t whine about it, just go on with it.

Footnotes

1. there are good reas­ons for this, but they are his­tor­ical so no-​​one cares any­way. Except me. But not today [back]
2. Is that just cheap or good for cus­tom­ers, hard to say [back]
3. and yes, I have used these devices when I was at University, they really did exist [back]

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