Configuring A Modem To Work With Cisco Routers
By Chris Bryant, CCIE #12933
With all the high-speed connection methods available today, it's easy to forget that the good old modem is still a valid way to connect to a Cisco router via dialup. Many of today's networks still use modems to do so, especially as a backup method, so it's important for the exam and for your work on production networks to understand how to configure such a connection.
The basic operation of a modem is simple if you know what modem actually means. It's an abbreviation of modulator / demodulator, and that's exactly what a modem does. Modulation occurs when the modem changes a computer's digital signal into analog signals. This allows the signal to be sent over basic telephone lines. Once the signal reaches the other side of the connection, demodulation changes the analog signal back into digital signals. This entire process is transparent to the computers sending and receiving the digital signals.
The good news is that the use of the telephone company's analog lines requires no special equipment or high-speed lines. The bad news is that the data transfer speed will generally be low.
Typically, you're going to use a DB-25 cable for DTE-DCE connections. The interface is officially called the EIA/TIA 232 interface, but it's still referred to by some as the RS232 interface (its original name). The name of the cable itself tells us there are 25 pins, but only eight of these pins are going to be used in a DTE-DCE connection. These eight pins are grouped according to task.
The data transfer pins:
Pin 2 - Transmit Data (TxD) - DTE uses this to send data to DCE. Sometimes referred to as TD.
Pin 3 - Receive Data (RxD) - DCE uses this to send data to DTE. Sometimes referred to as RD.
Pin 7 - Ground - (GRD) - Serves as reference point for other signals
The modem control pins:
Pin 6 - Data Set Ready (DSR) - DCE is ready.
Pin 8 - Carrier Detect (CD) - DCE detects carrier signal with remote DCE
Pin 20 - Data Terminal Ready (DTR) - DTE tells DCE that it can take a call.
The hardware flow control pins:
Pin 4 - Request To Send (RTS) - The DTE has buffer space available and is requesting the DCE to send it more data.
Pin 5 - Clear To Send (CTS) - The DCE has buffer space available and is requesting the DTE to send it more data.
Physically Connecting The Modem To The Router
In production networks, you're generally going to see a modem connected to the asynchronous interface or the auxiliary port. However, a modem can also be connected to the console port or a serial interface!
AUX port: Use a rolled cable and a RJ45-DB25 DCE modem adapter. Connect the adapter to the EIA-TIA 232 interface, the rolled cable to the adapter, and the other end of the rolled cable straight into the AUX port. Not all AUX ports are RJ45, so check your router before buying the cables.
Serial interface: Again, this is a situation where you will need to check your router documentation, because not all Serial interfaces will accept the following configuration. The command physical-layer async allows a serial interface to act as an asynchronous interface.
R1(config)#int serial0
R1(config-if)# physical-layer async
Console Port: You're not going to see a modem connected to a console port very often. This port really isn't built for modem usage, and there are inherent problems with using this port for modem connections. You can use the same steps for the AUX port connection here, but I recommend you stay away from using the console port for modems.
Some modem documentation mentions that having a modem connected to the console port allows you to perform Cisco password recovery remotely, but don't forget that those procedures requires someone at the router as well. Additionally, EXEC sessions do not terminate automatically when the connection to the mode drops. The user must terminate the connection properly or a security gap is created. There's also no reverse Telnet capabilities when connecting to a modem on the console port.
Working with modems is a lot like working with network design - there's no "one configuration fits all" solution. Having said that, I'm going to list some of the more common router configuration commands here that will help you pass the exam and allow you to get started with modems, but this is not a complete list of modem and router config commands.
The basic config starts much like the VTY line config to allow users to Telnet in. We'll configure line 12 with the login command and a password.
166(config)#line 12
166(config-line)#login
166(config-line)#password CCNP
For flow control, we've got two choices. The recommended selection is hardware flow control, also known as CTS/RTS flow control in reference to the pins used in this version of flow control. The use of software flow control, also called Xon/Xoff flow control, is not recommended by Cisco or by many modem vendors because of its tendency to slow data transmission due to the sheer number of start (Xon) and stop (Xoff) bits.
166(config-line)#flowcontrol ?
NONE Set no flow control
hardware Set hardware flow control
software Set software flow control
166(config-line)#flowcontrol hardware
Now the maximum transmission speed between the router and modem must be configured. IOS Help illustrates that this single command sets both the transmission and receive speeds. What IOS Help doesn't tell you is that this speed is set in BPS.
166(config-line)#speed ?
<0-4294967295> Transmit and receive speeds
166(config-line)#speed 56000
The transport input command dictates what protocols can be transmitted through this line. We'll use the all option.
166(config-line)#transport input ?
all All protocols
lat DEC LAT protocol
mop DEC MOP Remote Console Protocol
nasi NASI protocol
none No protocols
pad X.3 PAD
rlogin Unix rlogin protocol
telnet TCP/IP Telnet protocol
v120 Async over ISDN
166(config-line)#transport input all
Different modem manufacturers may dictate a certain number of stop bits. Use the stopbits command to set this value.
166(config-line)#stopbits ?
1 One stop bit
1.5 One and one-half stop bits
2 Two stop bits
166(config-line)#stopbits 2
You may choose to have the modem make outgoing calls while accepting incoming calls, or simply accept incoming calls. Use the modem command to specify this capability. There are some other options here that you may find helpful for a particular modem type.
166(config-line)#modem ?
DTR-active Leave DTR low unless line has an active incoming connection or EXEC
Dialin Configure line for a modern dial-in modem
Host Devices that expect an incoming modem call
InOut Configure line for incoming AND outgoing use of modem
Printer Devices that require DSR/CD active
answer-timeout Set interval between the time the server raises DTR in
response to RING and the modem responds to CTS
autoconfigure Automatically configure modem on line
busyout Block calls to and from the modem
To assign PPP features such as CHAP to a line, use interface async X, with x representing the line number.
164(config)#int async 2
164(config-if)#encap ppp
164(config-if)#ppp authentication chap
164(config-if)#async mode ?
dedicated Line is dedicated as an async interface
interactive Line may be switched between interactive use and async interface
Typical async interface values: encapsulation, authentication, and async mode interactive / dedicated. (Use IOS Help to show the options for async interfaces - there are a LOT of them, but these are the most common.)
Most other values - including the line password, the login command, flow control, speed, modem inout, stopbits, and transport input - are configured on the line.
164(config)#line 2
164(config-line)#login
164(config-line)#password CCNP
164(config-line)#flowcontrol hardware
164(config-line)#speed 56000
164(config-line)#modem inout
164(config-line)#transport input all
164(config-line)#stopbits 1
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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