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Student here. Could someone please explain to me the difference between these two? I'm using a serial connection for this. Does the latter refer to one individual serial terminal session? Why would one use line configuration mode over global? Thanks, I appreciate any help.

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up vote 6 down vote accepted

Global configuration mode is for configuring global parameters. This is the default mode when you enter configuration mode.

Some commands will cause you to enter a more specific configuration mode. Line configuration mode (similar to interface configuration mode) is entered when you specify a line to configure. Line configuration mode is then where you enter configuration appropriate to that specific line.

When you are in a specific configuration mode, if you prompt for help (i.e. type "?") you will only be shown commands that are relevant to that configuration mode rather than commands that are relevant in global configuration mode.

Here is the output of line configuration mode from one of my test switches:

Test(config-line)#?
Line configuration commands:
  access-class            Filter connections based on an IP access list
  accounting              Accounting parameters
  authorization           Authorization parameters
  autocommand             Automatically execute an EXEC command
  autocommand-options     Autocommand options
  data-character-bits     Size of characters being handled
  databits                Set number of data bits per character
  default                 Set a command to its defaults
  domain-lookup           Enable domain lookups in show commands
  editing                 Enable command line editing
  escape-character        Change the current line's escape character
  exec                    Configure EXEC
  exec-banner             Enable the display of the EXEC banner
  exec-character-bits     Size of characters to the command exec
  exec-timeout            Set the EXEC timeout
  exit                    Exit from line configuration mode
  flowcontrol             Set the flow control
  full-help               Provide help to unprivileged user
  help                    Description of the interactive help system
  history                 Enable and control the command history function
  international           Enable international 8-bit character support
  ip                      IP options
  length                  Set number of lines on a screen
  location                Enter terminal location description
  logging                 Modify message logging facilities
  login                   Enable password checking
  modem                   Configure the Modem Control Lines
  monitor                 Copy debug output to the current terminal line
  motd-banner             Enable the display of the MOTD banner
  no                      Negate a command or set its defaults
  notify                  Inform users of output from concurrent sessions
  padding                 Set padding for a specified output character
  parity                  Set terminal parity
  password                Set a password
  privilege               Change privilege level for line
  refuse-message          Define a refuse banner
  rotary                  Add line to a rotary group
  rxspeed                 Set the receive speed
  session-timeout         Set interval for closing connection when there is no input traffic
  special-character-bits  Size of the escape (and other special) characters
  speed                   Set the transmit and receive speeds
  start-character         Define the start character
  stop-character          Define the stop character
  stopbits                Set async line stop bits
  terminal-type           Set the terminal type
  timeout                 Timeouts for the line
  transport               Define transport protocols for line
  txspeed                 Set the transmit speeds
  vacant-message          Define a vacant banner
  width                   Set width of the display terminal

Notice that while some of the commands are the same (or similar), many of the global configuration mode commands you might expect are not present, such as aaa, boot or cdp. This last would be present in interface configuration mode, but there is no CDP on the console so it is not present.

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Thank you. Distilling further, I guess my real question is what does "line" mean in this regard? I know serial console sessions have only 1 line, yet telnet can support 16. What is the point of defining multiple sessions and how does it affect the configuration state of the switch? – user1330287 Mar 22 at 22:26
    
A line can only support one active connection/session at a time. So console only has one line because you can't make two connections to a single console port. VTY lines (or virtual TTYs) are used for things like telnet or SSH and can often support multiple connections at the same time. This is why most Cisco devices support at least 5 VTYs (modern devices support at least 16). While often all VTYs are configured the same, this does not need to be the case. For instance, you could "reserve" some VTYs by placing higher restrictions on them (i.e. only allow the "super-admin" computers by ACL). – YLearn Mar 23 at 20:05

It doesn't matter if you are connected to the router via console, telnet, SSH, etc., you configure a specific feature in the mode for that specific feature.

If you need to configure global commands, you do that in global configuration.

If you are configuring interfaces, you do that in interface configuration.

If you are configuring lines, you do that in line configuration.

If you are configuring a routing protocol, you do that in router configuration.

Etc.

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