Cisco Certification Exam Review:
Using Modems With Cisco Routers
By Chris Bryant, CCIE #12933
Routers, PCs, and mainframes are DTEs. Modems and CSU/DSUs are DCEs.
To directly connect two DTEs, use a null modem cable. These cables have pins 2 and 3 cross over, making DTE-DTE communication possible.
And speaking of pins.....
The data transfer pins:
Pin 2 - TxD - Transmit Data - DTE uses this to send data to DCE.
Pin 3 - RxD - Receive Data - DCE uses this to send data to DTE.
Pin 7 - GRD - Ground - Serves as reference point for other signals.
The modem control pins:
Pin 6 - DSR - Data Set Ready - DCE is ready.
Pin 8 - CD - Carrier Detect - DCE detects carrier signal w/ remote DCE
Pin 20 - DTR - Data Terminal Ready - DTE tells DCE it can take a call
The hardware flow control pins:
Pin 4 - RTS - Request To Send - DTE has buffer space available, tells DCE to send it more data
Pin 5 - CTS - Clear To Send - DCE has buffer space available and is telling the DTE to send it more data
Using different lines to connect the modem to the router:
AUX port - Use a rolled cable and an RJ45-DB25 DCE modem adapter cable.
Serial port - Not generally a good idea; if IOS allows, use the physical-layer async command to have the serial interface act as an asynchronous interface.
Console port - Even less of a good idea. Technically, password recovery is possible via a remote connection to this port - BUT someone's still going to have to be physically present at the device.
Reverse telnet is used to initiate a Telnet session OUT an asynchronous line. When using reverse telnet, always add 2000 to the line number - the TCP base port for the router's individual line is 2000. You'll also see this in IP Host tables.
To begin a Reverse Telnet session: telnet 200.1.1.1 2009 (using line 9)
To suspend a Reverse Telnet session: <ctrl - shift - 6> <x>
To end a Reverse Telnet session: type disconnect
Building an IP Host table can make reverse telnet sessions easier, particularly on an access server. Here's a sample table:
ip host SW2 2006 100.1.1.1
ip host SW1 2005 100.1.1.1
ip host R4 2004 100.1.1.1
ip host R3 2003 100.1.1.1
ip host R1 2001 100.1.1.1
ip host R2 2002 100.1.1.1
Instead of entering telnet 100.1.1.1 2006, the user can now just enter "SW2" at the prompt.
Check the status of the lines with show line. If an asterisk appears next to a line, it's in use. Line 0, the CTY line, is the Console line. The AUX port is the first line listed after all the asynchronous lines. If you have eight asynchronous lines, the Console line is Line 0, the async lines will be 1 - 8, and the AUX line will be line 9. If there are 16 asynchronous lines, the Console line will still be line 0, but the AUX line will be 17. An eight-line async router is shown below, with the router being accessed via the Console line and five separate async lines.
164#show line
Tty Typ Tx/Rx A Modem
* 0 CTY - -
* 1 TTY 9600/9600 - -
* 2 TTY 9600/9600 - -
* 3 TTY 9600/9600 - -
4 TTY 9600/9600 - -
* 5 TTY 9600/9600 - -
* 6 TTY 9600/9600 - -
7 TTY 9600/9600 - -
8 TTY 56000/56000 - inout
9 AUX 9600/9600 - -
Values set on the lines include flowcontrol, speed, transport input, and stopbits.
Flowcontrol options: hardware, software, none. Hardware is preferred by Cisco and almost all modem manufacturers. flowcontrol hardware enables CTS/RTS flow control; flowcontrol software sends an unacceptable number of on and off bits.
transport input dictates which protocols will be transported through this line. Typical choices are all or none.
Theoretically, a Cisco router can autoconfigure a modem by using the modem autoconfigure discovery command on the line the modem is connected to (or will be).
The MODEMCAP is a set of AT commands that allows the router to autoconfigure many modems on the market today.
Cisco recommends you lock the DTE speed listed in the MODEMCAP.
Cisco documentation states that the lock DTE speed command is also referred to as buffered mode or port rate adjust.
To edit the MODEMCAP, use the modemcap edit command.
To view MODEMCAP entries, you can run either show modemcap or show running-configuration. (If you can't remember a specific show command, you can almost always see what you need to see somewhere in show run or show interface.)
Chat scripts are text strings that define the DTE - DCE interaction. Today's chat scripts can do a lot of things, but among the basics are defining the modem dialout commands, modem initialization, modem configuration, and failure detection.
To create a logical group of asynchronous lines, use the group-async command to create the logical interface, followed by the group range command.
164(con)#interface group-async 1
164(con)#group-range ?
<1-8> Start of range
tty Terminal controller
164(con)#group-range 1 ?
<1-8> End of range
tty Terminal controller
164(con)#group-range 1 8
A typical line configuration, along with command options:
164(config)#line 8
164(config-line)#flowcontrol ?
NONE Set no flow control
hardware Set hardware flow control
software Set software flow control
164(config-line)#flowcontrol hardware
164(config-line)#stopbits ?
1 One stop bit
1.5 One and one-half stop bits
2 Two stop bits
164(config-line)#stopbits 2
164(config-line)#password ?
0 Specifies an UNENCRYPTED password will follow
7 Specifies a HIDDEN password will follow
LINE The UNENCRYPTED (cleartext) line password
164(config-line)#password CCNP
164(config-line)#login
164(config-line)#transport input ?
all All protocols
none No protocols
pad X.3 PAD
rlogin Unix rlogin protocol
telnet TCP/IP Telnet protocol
udptn UDPTN async via UDP protocol
v120 Async over ISDN
164(config-line)#transport input all
164(config-line)#speed ?
<0-4294967295> Transmit and receive speeds
164(config-line)#speed 56000
164(config-line)#modem ?
CTS-Alarm Alarm device which only uses CTS for call control
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
164(config-line)#modem inout
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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