Introduction

A terminal or comm server commonly provides out-of-band access for multiple devices. A terminal server is a router with multiple, low speed, asynchronous ports that are connected to other serial devices, for example, modems or console ports on routers or switches.
The terminal server allows you to use a single point to access the console ports of many devices. A terminal server eliminates the need to configure backup scenarios like modems on auxiliary ports for every device. You can also configure a single modem on the auxiliary port of the terminal server, to provide dial-up service to the other devices when network connectivity fails.
This document shows how to configure a terminal server to access only the console ports on other routers through Reverse Telnet. Reverse Telnet allows you to establish a Telnet connection out on the same device you telnet from, but on a different interface. For more information on Reverse Telnet refer to Establishing a Reverse Telnet Session to a Modem.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Cabling

The Cisco 2509 - 2512 series routers use a 68-pin connector and breakout cable. This cable (CAB-OCTAL-ASYNC) provides eight RJ-45 rolled cable async ports on each 68-pin connector. You can connect each RJ-45 rolled cable async port to the console port of a device. The 2511 router allows for a maximum of 16 devices to be remotely accessible. In addition, the NM-16A or NM-32A high density async network modules are available for the Cisco 2600 and 3600 series routers to provide the same function. For more information on cabling refer to the Let's Connect:Your Serial Cable Guide and the Cabling Guide for Console and AUX Ports.
Note: The async ports from the 68-pin connector are data terminal equipment (DTE) devices. DTE to DTE devices require a rolled (null modem) cable and DTE to data circuit-terminating equipment (DCE) devices require a straight-through cable. The CAB-OCTAL-ASYNC cable is rolled. Therefore, you can connect each cable directly to the console ports of devices with RJ-45 interfaces. However, if the console port of the device to which you connect is a 25-pin interface (DCE), you must use the RJ-45 to 25-pin adapter marked "Modem" (to reverse the "roll") in order to complete the connection.
This table shows the port types for console and auxiliary ports on Cisco routers and switches:

Interface Type	DB25 Interface	RJ-45 Interface
Console	DCE	DTE
AUX	DTE	DTE

Design Strategy

Configure the terminal server so that you can access the terminal server from anywhere. In order to make the terminal server accessible, assign a registered public Internet address, and locate the server outside the firewall. When you do so, firewall issues do not interrupt your connection. You can always maintain connectivity to the terminal server and access the connected devices. If you are concerned about security, configure access lists to allow access only to the terminal server from certain addresses. For a more robust security solution, you can also configure server-based authentication, authorization, and accounting (AAA) for example, RADIUS or TACACS+. For more information on AAA, refer to Authentication, Authorization, and Accounting (AAA).
You can configure a modem on the auxiliary port of the terminal server for dial backup in the event your primary connection (through the Internet) goes down. Such a modem eliminates the need to configure a dial backup for each device. The terminal server is connected through its async ports to the console ports of the other devices. For more information on how to connect a modem to the AUX port, refer to Modem-Router Connection Guide.
Use the ip default gateway statement, and point to the the next hop router on the Internet. This command enables you to have connectivity to the terminal server through the Internet even if routing is not enabled. For example, the terminal server is in ROM monitor (ROMMON) mode as a result of a bad reboot after a power outage.

Configure

In this section, you are presented with the information to configure the features described in this document.
Note: To find additional information on the commands used in this document, use the Command Lookup Tool