What exactly do they mean by "the line"?

the_dude7 · February 2018

I've seen this term thrown around a lot in the textbooks I'm using as well as part of the Cisco IOS sometimes, but they don't give an actual definition for it. Like in the context of "line con (console) 0" or in the message "line protocol is up", or when you're asked to place an ACL on the line when configuring the RO community string for a lab.

Without extra context, does it just refer to the normal, main (Ethernet) connection to the device? The console connection? As opposed to the VTY lines when those are specified?

Line protocol in particular is confusing for me and it's not explained well. What exactly is it?

HCPS123 · February 2018

Currently studying for ICND1 as well at the moment so I reserve the right to be wrong on this but I believe "line" simply means that, the actual physical cable you are connecting to said router. "Line con 0" is referring to the setup the router shows to people who take the console cable and manually hook it up to the router to interface into the router. "Line vty" are referring to your virtual lines (cables), aka your wireless access to said router. Likewise line protocol is referring to the physical cables that are hooked up to your router.

Line protocol is part of a two step report (pair with line status) that shows up when you use your show command "show ip int brief". You often use this in verification/troubleshooting to determine what kind of error you're having. If both are down you're experiencing a layer 1 (physical) issue and should check to see if you have a bad cable or if they're plug in. If line protocol itself is down you're experiencing a layer 2 (data-link) error which means something is probably not configured right on your router.

If it's admin down that simply means you forgot to turn the line on, by default lines or interfaces if you will, are turned off and you have to go in and manually turn them on.

Placing an ACL on a line is a weird way of saying placing an ACL on an interfaces, interface and line from my understanding are the same thing (Note I mean they refer to the same thing, the physical cable; they are very different commands on the actual IOS).

Does that make sense?

Ismaeljrp · February 2018

Line is synonym for connection. Whether that's a physical connection or a logical one utilizing some protocol. At least that's the way I look at it.

the_dude7 · February 2018

Cool, I had a feeling it was along those lines... no pun intended.

Thanks guys.

dontstop · February 2018

It's a throwback name like how VTTY/VTY is just a "Virtual" TTY (https://en.wikipedia.org/wiki/Teleprinter). An example would be how people say you dial a phone number on your Android/iPhone mobile, even though there is no "Dial" anymore. I guess you could say VDial

What exactly do they mean by "the line"?

Comments