In which configuration mode is line aux 0 used?

The line aux 0 command is entered in Global Config mode on Cisco IOS devices.

SystemGlobal Config

line aux 0

Enters line configuration mode for the auxiliary (AUX) port, allowing configuration of console-like settings for remote out-of-band management via modem or terminal server.

Definition: line aux 0 is a Cisco IOS global config command. Enters line configuration mode for the auxiliary (AUX) port, allowing configuration of console-like settings for remote out-of-band management via modem or terminal server.

Overview

The `line aux 0` command is used in Cisco IOS global configuration mode to enter line configuration mode for the auxiliary (AUX) port. The AUX port is a physical serial interface on Cisco routers and some switches that provides out-of-band management access, typically used for remote console-like connectivity via a modem or terminal server. This command is essential for network engineers who need to configure backup management access to network devices, especially when in-band management (e.g., SSH, Telnet over the network) is unavailable due to network outages or misconfigurations.

The AUX port operates similarly to the console port but is designed for remote access; it can be connected to a modem for dial-in or to a terminal server for remote console access over a separate network. Configuring the AUX port involves setting parameters such as baud rate, flow control, and line-specific settings like exec timeout and password protection. This command is typically used during initial device setup or when establishing a reliable out-of-band management path.

In the broader network configuration workflow, after entering `line aux 0`, you can apply commands like `login`, `password`, `exec-timeout`, `speed`, and `flowcontrol` to secure and optimize the AUX connection. Compared to the console line (`line con 0`), the AUX line is often configured with modem control commands (e.g., `modem InOut`) to allow dial-in and dial-out. It is important to note that the AUX port is not present on all Cisco devices; for example, many Catalyst switches lack an AUX port.

The configuration applied to the AUX line is stored in the running configuration and can be saved to the startup configuration. Privilege level requirements: entering global config mode requires privileged EXEC mode (enable), and then the `line aux 0` command is executed. There is no impact on network traffic; it only affects management access.

Understanding the AUX line configuration is crucial for CCNA and CCNP candidates as it appears in exam topics related to device management and remote access.

Syntax·Global Config

line aux 0

When to Use This Command

Configuring a modem connected to the AUX port for dial-in remote management when the network is down.
Setting login authentication and password for AUX access to provide out-of-band management.
Adjusting line parameters such as baud rate, stop bits, or flow control for a terminal server connected to the AUX port.
Enabling exec-timeout to automatically disconnect idle AUX sessions for security.

Parameters

Parameter	Syntax	Required	Description
line-number	0	Required	Specifies the AUX line number. On most Cisco routers, there is only one AUX port, designated as line 0. Entering '0' selects the AUX port for configuration. There is no other valid value.

Command Examples

Basic AUX line configuration with password and exec-timeout

line aux 0
password cisco
login
exec-timeout 5 0

Router(config-line)#

The command enters AUX line configuration mode. 'password cisco' sets the login password to 'cisco'. 'login' enables password authentication at login. 'exec-timeout 5 0' sets the EXEC timeout to 5 minutes and 0 seconds.

Configuring AUX line for modem dial-in with transport input

line aux 0
modem InOut
transport input all
speed 115200

Router(config-line)#

'modem InOut' allows the modem to answer incoming calls and place outgoing calls. 'transport input all' permits all protocols (Telnet, SSH, etc.) over the line. 'speed 115200' sets the baud rate for the serial connection.

Understanding the Output

The 'line aux 0' command does not produce output; it changes the prompt to 'Router(config-line)#' indicating you are in line configuration mode. Subsequent configuration commands are applied to the AUX line. To verify settings, use 'show line aux 0' which displays line status, baud rate, modem state, and configured parameters.

Key fields include 'Line' (line number), 'Baud rate' (speed), 'Modem' (modem state: ready, CTS, etc.), and 'Exec timeout' (idle timeout). A properly configured AUX line should show 'Modem' as 'ready' and 'Exec timeout' as set. Watch for 'Modem' showing 'no CTS' indicating a cable or modem issue.

Configuration Scenarios

Configure AUX Port for Dial-In Modem Access

A network engineer needs to set up a backup management connection to a remote router via a dial-up modem. The AUX port will be connected to an external modem for out-of-band access when the primary network is down.

Topology

R1(AUX)---RS-232---Modem---PSTN---Remote Admin

Steps

1.Step 1: Enter global configuration mode: Router> enable
2.Step 2: Enter AUX line configuration: Router# configure terminal
3.Step 3: Set the line speed to match the modem: Router(config)# line aux 0
4.Step 4: Configure modem control for dial-in and dial-out: Router(config-line)# speed 9600
5.Step 5: Set exec timeout to 10 minutes: Router(config-line)# modem InOut
6.Step 6: Enable password authentication: Router(config-line)# exec-timeout 10 0
7.Step 7: Set the login password: Router(config-line)# login
8.Step 8: Configure transport input to allow Telnet (optional): Router(config-line)# password cisco
9.Step 9: Exit and save configuration: Router(config-line)# transport input telnet
10.Router(config-line)# end
11.Router# copy running-config startup-config

Configuration

! Full IOS config block
Router> enable
Router# configure terminal
Router(config)# line aux 0
Router(config-line)# speed 9600
Router(config-line)# modem InOut
Router(config-line)# exec-timeout 10 0
Router(config-line)# login
Router(config-line)# password cisco
Router(config-line)# transport input telnet
Router(config-line)# end
Router# copy running-config startup-config

Verify: Use 'show line aux 0' to verify configuration. Expected output includes line speed, modem state, and login settings. Also test by dialing into the modem and establishing a Telnet session.

Watch out: A common mistake is forgetting to set the modem control command 'modem InOut' which allows both incoming and outgoing calls. Without it, the modem may not answer incoming calls.

Configure AUX Port for Terminal Server Access

In a data center, a terminal server is used to provide out-of-band console access to multiple routers. The AUX port on each router is connected to the terminal server for remote management.

Topology

Terminal Server---RS-232---R1(AUX) (and other routers)

Steps

1.Step 1: Enter global configuration mode: Router> enable
2.Step 2: Enter AUX line configuration: Router# configure terminal
3.Step 3: Disable modem control (since no modem is used): Router(config)# line aux 0
4.Step 4: Set exec timeout to 0 (no timeout) for persistent connection: Router(config-line)# no modem
5.Step 5: Enable password authentication: Router(config-line)# exec-timeout 0 0
6.Step 6: Set the login password: Router(config-line)# login
7.Step 7: Configure transport input to allow Telnet: Router(config-line)# password cisco
8.Router(config-line)# transport input telnet
9.Router(config-line)# end
10.Router# copy running-config startup-config

Configuration

! Full IOS config block
Router> enable
Router# configure terminal
Router(config)# line aux 0
Router(config-line)# no modem
Router(config-line)# exec-timeout 0 0
Router(config-line)# login
Router(config-line)# password cisco
Router(config-line)# transport input telnet
Router(config-line)# end
Router# copy running-config startup-config

Verify: Use 'show line aux 0' to verify that modem control is disabled and exec-timeout is set to 0. Also verify connectivity from the terminal server by attempting to Telnet to the router's AUX IP (if configured) or via reverse Telnet.

Watch out: A common mistake is leaving modem control enabled when no modem is attached, which can cause the AUX line to hang waiting for modem signals. Always use 'no modem' when connecting directly to a terminal server.

Troubleshooting with This Command

When troubleshooting AUX port issues, the primary command is `show line aux 0`. This command displays the current configuration and status of the AUX line, including line speed, modem control settings, exec timeout, and whether login is enabled. Healthy output shows the expected parameters: speed matching the connected device, modem control set appropriately (e.g., 'inout' for modem, 'none' for terminal server), and login enabled with a password.

Problem indicators include: 'Timeout' values set too low causing premature disconnection, 'Modem' state showing 'CTS lost' or 'DSR low' indicating cable or modem issues, or 'Login' disabled allowing unauthorized access. Common symptoms: inability to establish a connection via the AUX port, frequent disconnections, or no response when dialing in. A step-by-step diagnostic flow: 1) Check physical connections: ensure the AUX cable is securely connected and the modem/terminal server is powered on. 2) Verify line configuration with `show line aux 0`. 3) Check for hardware issues: use `show controllers aux 0` to see if the interface is up. 4) Test with a loopback plug to isolate the router from the modem. 5) Correlate with debug commands: `debug modem` or `debug line` to see modem control signals and line activity.

For example, if `debug modem` shows 'Modem: RING' but no answer, the modem control command may be missing. Also, check the exec-timeout setting: if set too low, the session may drop after inactivity. Another useful command is `show users` to see active AUX sessions.

If the AUX line is in use, you may see an entry with 'aux' as the line. To clear a hung session, use `clear line aux 0`. Remember that the AUX port is asynchronous serial and typically uses a DB-25 or RJ-45 connector (depending on the router model).

Always ensure the cable pinout matches the device (e.g., Cisco AUX cable is rolled, similar to console).

CCNA Exam Tips

CCNA exam may ask which command enters AUX line configuration mode: 'line aux 0'.

Remember that AUX line is used for out-of-band management, typically via modem, and is configured similarly to console line.

Know that 'transport input' on AUX line can restrict protocols; 'transport input all' allows Telnet and SSH.

Be aware that 'exec-timeout' applies to all lines; setting it to 0 0 disables timeout (not recommended for security).

Common Mistakes

Mistake 1: Using 'line aux 0' in global config but forgetting to apply 'login' or 'password', leaving the line unsecured.

Mistake 2: Setting 'speed' too high for the connected modem or terminal server, causing connection issues.

Mistake 3: Confusing AUX line with console line; AUX is typically used for remote dial-in, not direct console access.

line aux 0 vs exec-timeout [min] [sec]

Both commands relate to Cisco IOS line configuration, often causing confusion because 'line aux 0' enters a specific line configuration context, while 'exec-timeout' is a command used within that context to manage session timeouts. Understanding their distinct roles is crucial for proper remote management and security.

Aspect	line aux 0	exec-timeout [min] [sec]
Scope	Affects only the auxiliary line (AUX port).	Applies to the current line (e.g., AUX, console, vty).
Configuration Mode	Global configuration mode.	Line configuration mode (e.g., line aux 0 prompt).
Persistence	Persistent; saved in running-config.	Persistent; saved in running-config under the line.
Precedence	Must be issued before other line-specific commands.	Set within the line; overrides default timeout.
Typical Use	Enter AUX port configuration for modem/terminal server access.	Define automatic logout after inactivity (security).

Use line aux 0 when you need to configure settings specific to the auxiliary port, such as modem control or speed.

Use exec-timeout [min] [sec] when you need to enforce an inactivity timeout on a line to automatically log out idle users.

Platform Notes

In IOS-XE, the `line aux 0` command syntax and behavior are identical to classic IOS. However, on some IOS-XE platforms (e.g., ISR 4000 series), the AUX port may be virtual or not present; check hardware documentation. In NX-OS, there is no direct equivalent; management is typically done via the management Ethernet interface (mgmt0) or console port.

The AUX port is not supported on NX-OS devices. On ASA firewalls, the AUX port is also not present; management is via console or management interface. For IOS-XR, the command does not exist; out-of-band management is via console or auxiliary ports on some hardware, but configuration is done differently (e.g., using `line console` or `line aux` in XR is not standard).

In older IOS versions (12.x), the AUX line configuration was similar, but some commands like `modem InOut` were introduced later. In IOS 15.x and 16.x, the syntax remains consistent. Some newer routers (e.g., ISR 1100) may not have a physical AUX port; instead, they use a USB console port.

Always verify hardware capabilities before relying on AUX configuration.

Related Commands

exec-timeout [min] [sec]

Sets the inactivity timeout for an EXEC session on a line, automatically logging out idle users to free up resources and enhance security.

show line

Displays line configuration and status information for console, auxiliary, and vty lines, used to verify line settings and monitor user connections.

Practice for the CCNA 200-301

Test your knowledge with practice questions covering all CCNA 200-301 exam domains.

Practice CCNA 200-301 Questions