In preparation of your CCNA exam, we want to make sure we cover the various concepts that we could see on your Cisco CCNA exam. So to assist you, below we will discuss one of the more difficult CCNA concepts; Terminal Server Setup. As you progress through your CCNA exam studies, I am sure with repetition you will find this topic becomes easier. So even though it may be a difficult concept and confusing at first, keep at it as no one said getting your Cisco certification would be easy!
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, such as 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. Using a terminal server avoids the need for configuring backup scenarios such as modems on auxiliary ports for every device. You can also configure a single modem on the auxiliary port of the terminal server, thus providing dial-up service to the other devices during a network connectivity failure.
This document shows how to configure a terminal server to access only the console ports on other routers using Reverse Telnet. Reverse Telnet allows you to Telnet out from a device you are telnetting from, but on a different interface. For more information on Reverse Telnet refer to Establishing a Reverse Telnet Session to a Modem.
The Cisco 2509 – 2511 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 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 Serial Cable Guide and the Cabling Guide for RJ-45 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. Since the CAB-OCTAL-ASYNC cable is itself rolled, you can connect each cable directly to the console ports of devices with RJ-45 interfaces. However, if the console port of the device you are connecting to is a 25 pin interface (DCE) use the RJ-45 to 25 pin adapter marked “Modem” (to reverse the “roll”) to complete the connection.
Port types for console and auxiliary ports on Cisco routers and switches are:
Configure the terminal server so that it is accessible from anywhere by giving it a registered public Internet address, and by locating it outside the firewall so that firewall issues will not interrupt your connection. This ensures that you can always maintain connectivity to the terminal server and have access to the connected devices. If you are concerned about security, you may want to configure access lists to only allow access to the terminal server from certain addresses. You can also configure server-based authentication, authorization, and accounting (AAA) such as RADIUS or TACACS+ for a more robust security solution. For more information on AAA refer to the Cisco AAA Implementation Case Study.
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. This eliminates the need to configure a dial backup for each device, as the terminal server is connected through its async ports to the console ports of the other devices. For more information on connecting a modem to the AUX port, refer to Modem-Router Connection Guide.
Use the ip default gateway statement pointing to the the next hop router on the Internet. This allows 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.
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 ( registered customers only).
This document uses the network setup shown in the diagram below.
This document uses the configuration shown below.
ip host – Used to define the static host’s name-to-address mapping in the host cache. To remove the name-to-address mapping, use the no form of this command.
- ip host name