-
Dial Solutions Configuration Guide
-
About The Cisco IOS Software Documentation
- Part 1: Business Applications and Scenarios
-
Part 2: Dial-In Port Set-Up
-
Overview of Lines, Interfaces, and Controllers
-
Configuring Modem Support and Asynchronous Devices
-
Managing Modems
-
Configuring Terminal Characteristics for Dial Sessions
-
Setting Up ISDN Basic Rate Service
-
Configuring Synchronous Serial Ports
-
Configuring Channelized E1 and Channelized T1
-
Configuring Special ISDN Signaling
-
-
Part 3: Dial-In Terminal Service and Remote Node Configuration
-
Configuring Dial-In Terminal Services
-
Configuring Protocol Translation and Virtual Async Devices
-
Configuring the Cisco PAD Facility for X.25 Connections
-
Configuring Media-Independent PPP and Multilink PPP
-
Configuring Asynchronous PPP and SLIP
-
Configuring V.120 Remote Node Access
-
Configuring AppleTalk Remote Access
-
- Part 4: Dial Authentication
- Part 5: Dial-on-Demand Routing
- Part 6: Dial Backup
- Part 7: Dial-Out Modem Pooling
- Part 8: Large-Scale Dial Solutions
- Part 9: Cost-Control Solutions
- Part 10: Virtual Private Dialup Networks
- Part 11: Other Network Traffic on ISDN Channels
- Part 12: Dial-Related Addressing Services
-
Table Of Contents
Configuring Modem Support and Asynchronous Devices
Modem Support Configuration Task List
Configure Additional Modem Control Parameters
Asynchronous Configuration Task List
Specify an Asynchronous Interface
Configure Group and Member Asynchronous Interfaces
Configuring Dual-Purpose Ports
Assign IP Addresses for Local Devices
Configure the Interface Addressing Method for Remote Devices
Configure Asynchronous Serial Encapsulation
Configure Dedicated or Interactive PPP and SLIP Sessions
Enable Routing on Asynchronous Interfaces
Configure Support for Extended BOOTP Requests
Monitor and Maintain Asynchronous Devices
Configure Chat Scripts for Asynchronous Lines
Suggested Chat Script Naming Conventions (for Dial Scripts only)
Configure the Line to Activate Chat Scripts
Start a Chat Script Manually on an Asynchronous Line
Configure Additional Asynchronous Features
Establish and Control the EXEC Process
Configure the Auxiliary (AUX) Port
Configure the Line to Automatically Detect the Incoming Protocol and Start a Session
Define a Command String for Automatic Execution
Specify Decimal TCP Port Numbers when Connecting to Lines
Asynchronous Configuration Examples
Restricting Access on the Asynchronous Interface Example
Address Pooling on Asynchronous Interfaces Examples
Group and Member Asynchronous Interfaces Examples
Dedicated Asynchronous Interface Configuration Example
IP-SLIP—Asynchronous Interface Example
IPX-PPP—Loopback Interface Example
IPX-PPP—Using Dedicated IPX Network Numbers for Each Interface Example
IPX-PPP over X.25 to an IPX Network on VTY Lines Example
Asynchronous Routing and Dynamic Addressing Configuration Example
TCP Header Compression Configuration Example
Conserving Network Addresses Using the IP Unnumbered Feature Example
Configuring Routing on a Dedicated Dial-In Router Example
Configuring an Asynchronous Interface as the Only Network Interface Example
Configuring an Interface Example
Remote Network Access Using PPP—A Basic Configuration Example
Remote Network Access Using PPP—Routing IP Example
Remote Network Access—Leased Line with Dial-Backup Using PPP Example
Configuring Modem Support and Asynchronous Devices
This chapter describes how to configure the Cisco IOS software for line, terminal, and modem connections. Cisco devices have four types of lines: console, auxiliary, asynchronous, and virtual terminal lines. Different routers have different numbers of these line types. Refer to the hardware or software configuration guide that shipped with your device for exact line numbering schemes.
For a complete description of the commands in this chapter, refer to the "Modem Support and Asynchronous Device Commands" chapter of the Dial Solutions Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
The following sections are provided in this chapter:
•
Modem Support Configuration Task List
•
•
•
•
•
•
General Concepts
Asynchronous ports can be on the outside of your access server (for example, the Cisco AS2511-RJ) or on the inside of your device (for example, the Cisco AS5200). In both cases, external or internal modems connect to these interfaces. The remote clients referenced in this chapter are making analog calls in to the network via asynchronous ports.
To enable clients to dial in, you configure two components of each asynchronous port: lines and interfaces. Asynchronous interfaces correspond to physical terminal (TTY) lines. For example, asynchronous interface 1 corresponds to TTY line 1.
Generally, commands entered in asynchronous interface mode enable you to configure protocol-specific parameters for asynchronous interfaces, whereas commands entered in line configuration mode permit you to configure the physical aspects for the same port. In , which shows the show line output on a Cisco 2511, TTY line 1 corresponds with asynchronous interface 1, TTY line 16 corresponds with asynchronous interface 16, and so on.
Figure 35 Show Line Output—Comparing TTY Lines to Asynchronous Interfaces
Asynchronous Modem Lines
Asynchronous line configuration commands configure ports for the following options:
•
•
•
•
To enter line configuration mode, first connect to the console port of the access server and enter privileged EXEC mode. Then enter global configuration mode and finally enter line configuration mode for the asynchronous lines that you want to configure. The following example shows the process of entering line configuration mode for lines 1 through 16:
router> enablerouter# configure terminalrouter(config)# line 1 16router(config-line)#Asynchronous Interfaces
Generally, interfaces enable the Cisco IOS software to use routing functions. Specifically, you configure asynchronous interfaces to support PPP connections. You configure interfaces on an access server for the following functions:
•
•
•
•
•
•
Line Numbering Issues
The TTY line numbering scheme used by your access server or router is specific to your product and its hardware configuration. Refer to the product-specific documentation that came with your product for line numbering scheme information.
For example the Cisco AS5200 access server has TTY lines that map directly to integrated modems as shown in . The TTY lines 1 through 24 directly connect to modems 1/0 through 1/23, which are installed in the first chassis slot in this example. The TTY lines 25 through 48 directly connect to modems 2/0 through 2/23, which are installed in the second chassis slot. For more information about the Cisco AS5200's slot numbering scheme, refer to the Cisco AS5200 Universal Access Server Software Configuration Guide.
Group Asynchronous Interfaces
To configure multiple asynchronous interfaces at the same time (with the same parameters), you can assign each asynchronous interface to a group and then configure the group. Configurations throughout this guide configure group asynchronous interfaces, rather than configuring each interface separately.
Note
To configure a group asynchronous interface, specify the group async number (an arbitrary number) and the group range (beginning and ending asynchronous interface number). The following example shows the process of creating and configuring a group asynchronous interface for asynchronous interfaces 1 through 48 on a Cisco AS5200 access server, which is loaded with 48 V.34 Microcom modems:
router(config)# interface group-async 1router(config-if)# ip unnumbered ethernet 0router(config-if)# encapsulation ppprouter(config-if)# async mode interactiverouter(config-if)# ppp authentication chap paprouter(config-if)# peer default ip address pool defaultrouter(config-if)# group-range 1 48Building configuration...router(config-if)#Modem Support Configuration Task List
You can perform the tasks in the following sections to set up externally or internally connected modems. The first three tasks are required. The last task is optional.
•
Attach the Modems
Before configuring lines on a Cisco access server, you must attach your modems to the asynchronous ports. To attach a modem, refer to the user guide or installation guide that accompanied your access server.
Some Cisco access servers (such as the Cisco AS5200 universal access server), have integrated modems and do not require that you attach an external modem. If you have an access server with integrated modems, you can skip this section and proceed to the next section "Configure the Modem Lines."
Configure the Modem Lines
You must configure the lines to which you attach modems and allow dialin access. To configure lines, enter line configuration mode for the specific lines you need to configure. The following example shows lines 1 through 16 being configured on a Cisco 2511 access server (remember, bold screen font indicates what you type):
router> enablePassword: <password>router# configure terminalEnter configuration commands, one per line. End with CNTL/Z.router(config)# line 1 X (X = the highest number of lines the router has; 8 or 16)router(config-line)#After you enter line configuration mode for the lines to which your modems are attached, configure the lines using the required commands listed in the following task table. These are the most common commands necessary for basic modem communications.
1
speed 115200
or
speed 57600
or
speed 38400
Sets line speed to the highest common speed for the modem and the access server port. See for a list of modem transmission rates and line speeds you should set on the access server.1
2
flowcontrol hardware
Sets RTS/CTS flow control on the line.
3
modem inout
Configures the line to drop the connection when the Carrier Detect (CD) signal is lost (cycle DTR to close the connection). The modem dialin command can be used instead of the modem inout command to enhance security because the modem dialin command restricts outgoing connections from the access server to the modem. Use the modem inout command during setup so that you can configure the modem from the access server, and change to the modem dialin command when testing is completed. Use the modem host command to attach or connect to a printer or host device.
1 Cisco Systems assumes that your modem supports flow control. Refer to your modem documentation to learn whether your modem supports flow control, and to learn the maximum transmission speed of your modem.
Table 6 Matching the Access Server Line Speed to Your Modem Speed
9600
38400
14400
57600
28800
115200
Configure the Modems
This section describes the tasks required to configure modems that are externally attached to the access server:
•
•
•
When you configure modems to function with your access server, you must provide initialization strings and other settings on the modem to tell it how to function with the access server.
For modem configuration information about specific access servers that have integrated modems, refer to the section "Checking Other Modem Settings" later in this chapter.
Communicating with the Modem
This section assumes you have already physically attached the modem to the access server. If not, refer to the user guide or installation and configuration guide for your access server for information about attaching modems.
Before you can configure the modem, you must establish communication with it, which requires terminal access to the modem's command environment. The process of manually configuring a modem consists of the following tasks:
1
2
3
Establishing a Direct Telnet Session to the Modem
You communicate with the modem by establishing a direct Telnet session from the access server's asynchronous line, which is connected to the modem.
Note
This section explains how to establish a direct Telnet session with an external modem (existing outside the router) that is connected through an asynchronous port. Internal or integrated modems, such as used with the Cisco AS5200, are connected to through a direct connect session issued from the Cisco IOS software, which is described later in this section.
To establish a direct Telnet session to an external modem, determine the IP address of your LAN (Ethernet) interface, then enter a Telnet command to port 2000 + n on the access server, where n is the line number to which the modem is connected. For example, to connect to the modem attached to line 1, enter the following command from an EXEC session on the access server:
router# telnet 172.16.1.10 2001Trying 172.16.1.10, 2001 ... OpenThis example enables you to communicate with the modem on line 1 using the AT (attention) command set defined by the modem vendor.
Timesaver
Use the ip host configuration command to simplify direct Telnet sessions with modems. The ip host command maps an IP address of a port to a device name. For example, the modem1 2001 172.16.1.10 command enables you to enter modem1 to initiate a connection with the modem, instead of repeatedly entering telnet 172.16.1.10 2001 each time you want to communicate with the modem.
If you are unable to connect to the modem, check the following:
1
2
3
Modem state: IdleModem hardware state: CTS noDSR DTR RTS4
5
Testing the Modem Connection
After you make a direct Telnet connection to the modem, you need to test the connection. Send the modem the AT command to request its attention. It should respond with OK. For example:
atOKIf the modem does not reply to the AT command, check the following:
1
2
Suspending and Terminating Telnet Sessions
If you are connected to an external modem, the direct Telnet session must be terminated before the line can accept incoming calls. If you do not terminate the session, it will be indicated in the output of the show users command when it returns a modem state of ready if the line is still in use. If the line is no longer in use, the output of the show line value command will return a state of idle. For routers that support integrated modems (for example, the Cisco AS5200), calls will not be interrupted unless you enter an AT command that requires extra processing power from the modem's CPU, such as the ATH command.
Terminating the Telnet session requires first suspending it, then disconnecting it. To suspend a Telnet session, enter the escape sequence Ctrl-Shift-6 x (press Control-Shift-6, let go, then press x). Enter the disconnect EXEC command to terminate the Telnet session.
Note
To suspend and then disconnect a Telnet session, perform the following steps:
Step 1
- suspend keystroke -router#Step 2
router# whereConn Host Address Byte Idle Conn Name* 1 172.16.1.10 172.16.1.10 0 0 172.16.1.102 172.16.1.11 172.16.1.11 0 12 modem2Step 3
router# telnet modem2Trying modem2 (172.16.1.11, 2002) ... Open- suspend keystroke -router#Step 4
router# disconnect line 1Closing connection to 172.16.1.10 [confirm] yrouter# disconnect line 2Closing connection to 172.16.1.11 [confirm] yrouter#
Note
After you have established and tested the connection to the modem, you can proceed with the next section "Automatically Configuring Your External Modem."
Automatically Configuring Your External Modem
The Cisco IOS software can issue initialization strings automatically for most types of modems externally attached to the access server. A modem initialization string is a series of parameter settings that are sent to your modem to configure it to interact with the access server in a specified way. The Cisco IOS software defines seven initialization strings that have been found to properly initialize most modems so that the modems function properly with Cisco access servers. These initialization strings have the following names:
•
•
•
•
•
•
•
Note
If you do not know which of these modem strings is appropriate for your modems, issue the modem autoconfigure discovery line configuration command, as shown in the following example:
router# configure terminalrouter(config)# line 1 16router(config-line)# modem autoconfigure discoveryrouter(config-line)# Ctrl-Zrouter# copy running-config startup-configThe Cisco IOS software first tries the first of these strings to see if the modem initializes properly. If not, the Cisco IOS software cycles to the next string and repeats the process until the appropriate string is found. If none of the strings properly initializes the modem, you must manually configure the modem (refer to "Manually Configuring Your External Modem" later in this chapter).
If you know that your modem can be configured using an initialization string from one of these scripts, you can issue the modem autoconfigure type type command, where type is one of the strings in the preceding list. If you list a specific modem type, initialization proceeds more quickly.
To display the list of modems for which the router has modem string entries, issue the show modemcap command. You can change a modem value that was returned from the show modemcap EXEC command. For example, you might want to add the factory default, &F, entry to the configuration file. To do this, enter the modemcap edit modem-name attribute value line configuration command. Configure one attribute of one modem at a time.
The following example shows how to enter line configuration mode and issue the modem autoconfigure type type command for a US Robotics Sportster modem:
router(config-line)# modem autoconfigure type usr_sportsterFor more information about the recommended strings for any type of modem, refer to the section "Sample Modem Strings" in the appendix "Configuring Modem Support and Chat Scripts" in the Dial Solutions Command Reference. If you have access to Cisco Connection Online (CCO), you can also access the following URL for more information:
http://www.cisco.com/warp/public/76/4.html
Note
Manually Configuring Your External Modem
If you cannot configure your modem automatically, you must configure it manually. The following sections describe how to configure your externally attached modem manually:
•
•
Configuring Modem Initialization Strings
This section describes how to determine and issue the correct initialization string for your modem and configure your modem with it.
Modem command sets vary widely. Although most modems use the Hayes command set (prefixing commands with AT), Hayes-compatible modems do not use identical AT command sets.
Refer to your modem manufacturer's documentation to learn how to examine the current and stored configuration of the modem you are using. Generally, you enter AT commands such as &v, i4, or *o to view, inspect, or observe the settings.
Note
A sample modem initialization string for a US Robotics Courier modem is as follows:
&b1&h1&r2&c1&d3&m4&k1s0=1Modem initialization strings enable the following functions:
•
•
•
•
•
Timesaver
Initialization strings for other modems are listed in the appendix "Configuring Modem Strings and Chat Scripts" in the Dial Solutions Command Reference and on Cisco Connection Online (CCO), in Tech Tips (http://www.cisco.com). Note that these URLs are subject to change without notice.
Note
The port speed must not change when a session is negotiated with a remote modem. If the speed of the port on the access server is changed, you must establish a direct Telnet session to the modem and send an AT command so that the modem can learn the new speed.
Modems differ in the method they use to lock the EIA/TIA-232 (serial) port speed. In the modem documentation, vendors use terms such as, port-rate adjust, speed conversion, or buffered mode. Enabling error correction often puts the modem in the buffered mode. Refer to your modem documentation to see how your modem locks speed (check the settings &b, \j, &q, \n, or s-register settings).
Ready-To-Send (RTS) and Clear-To-Send (CTS) signals must be used between the modem and the access server to control the flow of data. Misconfiguring flow control for software or setting no flow control can result in hung sessions and loss of data. Modems differ in the method they use to enable hardware flow control. Refer to your modem documentation to see how to enable hardware flow control (check the settings &e, &k, &h, &r, or s-register).
The modem must use the data carrier detect (DCD) wire to indicate to the access server when a session has been negotiated and is established with a remote modem. Most modems use the setting &c1. Refer to your modem documentation for the DCD settings used with your modem.
The modem must interpret a toggle of the Data Terminal Ready (DTR) signal as a command to drop any active call and return to the stored settings. Most modems use the settings &d2 or &d3. Refer to your modem documentation for the DTR settings used with your modem.
If a modem is used to service incoming calls, it must be configured to answer a call after a specific number of rings. Most modems use the setting s0=1 to answer the call after one ring. Refer to your modem documentation for the settings used with your modem.
Checking Other Modem Settings
This section defines other settings that might be needed or desirable depending on your modem.
Error correction can be negotiated between two modems to ensure a reliable data link. Error correction standards include LAPM and MNP4. V.42 error correction allows either LAPM or MNP4 error correction to be negotiated. Modems differ in the way they enable error correction. Refer to your modem documentation for the error correction methods used with your modem.
Data compression can be negotiated between two modems to allow for greater data throughput. Data compression standards include V.42 bis and MNP5. Modems differ in the way they enable data compression. Refer to your modem documentation for the data compression settings used with your modem.
To establish a direct connect session to an internal or integrated modem (existing inside the router), such as required for Microcom modems in the Cisco AS5200, first open a directly connected session with the modem at-mode command then send an AT command to the specified modem. For example, the following example sends the AT command at\s to modem 1/1:
AS5200# modem at-mode 1/1You are now entering AT command mode on modem (slot 1 / port 1).Please type CTRL-C to exit AT command mode.at%vMNP Class 10 V.34/V.FC Modem Rev 1.0/85OKat\sIDLE 000:00:00LAST DIALNET ADDR: FFFFFFFFFFFFMODEM HW: SA 2W United States4 RTS 5 CTS 6 DSR - CD 20 DTR - RIMODULATION IDLEMODEM BPS 28800 AT%G0MODEM FLOW OFF AT\G0MODEM MODE AUT AT\N3V.23 OPR. OFF AT%F0AUTO ANS. ON ATS0=1SERIAL BPS 115200 AT%U0BPS ADJUST OFF AT\J0SPT BPS ADJ. 0 AT\W0ANSWER MESSGS ON ATQ0SERIAL FLOW BHW AT\Q3PASS XON/XOFF OFF AT\X0PARITY 8N ATInitializing the Modem
Refer to this section if you could not or chose not to initialize your modems automatically, as described in the "Automatically Configuring Your External Modem" section earlier in this chapter.
After the modem initialization string has been determined, perform the following steps to configure the modem. This example configures a U.S. Robotics Courier modem on line 1 (decimal number 2000 + line number 1 = 2001):
Step 1
router(config)# ip host modem1 2001 172.16.1.10router(config)# exitrouter#Step 2
router# telnet modem1Trying modem1 (172.16.1.10, 2001)... OpenStep 3
at&fOKStep 4
at&b1&h1&r2&c1&d3&m4&k1s0=1OKStep 5
at&wOK
Note
Step 6
- suspend keystroke -router# disconnectClosing connection to modem1 [confirm] yrouter#
Timesaver
The script-reset line configuration command can automate the configuration of your modems. See the "Technical Tips" section on CCO for more information.
Testing the Dial-In Connection
The access server and modem are now correctly configured for dial-in access. Before configuring any additional protocols for the line (such as SLIP, PPP, or ARA), test the dial-in connection.
Note
The following is an example of a successful connection from a PC using a U.S. Robotics Courier modem to dial in to a Cisco 2500 series access server:
at&f&c1&d3&h1&r2&b1&m4&k1&wOKatdt9,5551234CONNECT 14400/ARQ/V32/LAPM/V42BISUser Access VerificationUsername: janedoePassword:router>Configure Additional Modem Control Parameters
Cisco routers use six EIA/TIA-232 signals for each port, so one 50-pin telco, RJ-11, or RJ-45 connector can support eight sessions. The router can support the most popular forms of modem control and hardware flow control, as well as high-speed dial-up modems.
The EIA/TIA-232 output signals are Transmit Data (TXDATA), Data Terminal Ready (DTR), and Ready To Send (RTS, 2500 only). The input signals are Receive Data (RXDATA), Clear to Send (CTS), and RING. The sixth signal is ground. Depending on the type of modem control your modem uses, these names may or may not correspond to the standard EIA/TIA-232 signals.
Dial-up modems that operate over normal telephone lines at speeds of 28800 bits per second (bps) use hardware flow control to stop the data from reaching the host by toggling an EIA/TIA-232 signal when their limit is reached.
In addition to hardware flow control, modems require special software configuring. For example, they must be configured to create an EXEC session when a user dials in and to hang up when the user exits the EXEC. These modems also must be configured to close any existing network connections if the telephone line hangs up in the middle of a session.
The Cisco IOS software supports hardware flow control on its CTS input signal, which is also used by the normal modem handshake.
The following modem line characteristics and modem features are discussed in the following sections:
•
•
•
•
•
Signal and line state diagrams accompany some of the tasks in the following sections to illustrate how the modem control works. The diagrams show two processes:
•
•
through .)In the diagrams, the current signal state and the signal the line is watching are listed inside each box. The state of the line (as displayed by the show line EXEC command) is listed next to the box. Events that change that state appear in italics along the event path, and actions that the software performs are described within the ovals.
Figure 36 illustrates line states when no modem control is set. The DTR output is always high, and CTS and RING are completely ignored. The Cisco IOS software starts an EXEC session when the user types the activation character. Incoming TCP connections occur instantly if the line is not in use and can be closed only by the remote host.
Figure 36 EXEC and Daemon Creation on a Line with No Modem Control
Configure Automatic Dialing
With the dial-up capability, you can set a modem to dial the phone number of a remote router automatically. This feature offers cost savings because phone line connections are made only when they are needed—you only pay for using the phone line when there is data to be received or sent. To configure a line for automatic dialing, perform the following task in line configuration mode:
Using the modem dtr-active command causes a line to raise DTR signal only when there is an outgoing connection (such as reverse Telnet, NASI, or DDR), rather than leave DTR raised all the time. When raised, DTR potentially tells the modem that the router is ready to accept a call.
Automatically Answer a Modem
You can configure a line to answer a modem automatically. You also can configure the modem to answer the telephone on its own (as long as DTR is high), drop connections when DTR is low, and use its Carrier Detect (CD) signal to accurately reflect the presence of carrier. (Configuring the modem is a modem-dependent process.) Wire the modem's CD signal (generally pin-8) to the router's RING input (pin-22), and perform the following task in line configuration mode:
You can turn on the modem's hardware flow control independently to respond to the status of the router's CTS input. Wire CTS to whatever signal the modem uses for hardware flow control. If the modem expects to control hardware flow in both directions, you might also need to wire the modem's flow control input to some other signal that the router always has high (such as the DTR signal).
Figure 37 illustrates the modem dialin process with a high-speed dial-up modem. When the Cisco IOS software detects a signal on the RING input of an idle line, it starts an EXEC or autobaud process on that line. If the RING signal disappears on an active line, the Cisco IOS software closes any open network connections and terminates the EXEC facility. If the user exits the EXEC or the software terminates because of no user input, the line makes the modem hang up by lowering the DTR signal for five seconds. After five seconds, the modem is ready to accept another call.
Figure 37 EXEC Creation on a Line Configured for a High-Speed Modem
Support Dial-In and Dial-Out Modems
You can configure a line for both incoming and outgoing calls by performing the following task in line configuration mode:
Figure 38 illustrates the modem inout command. If the line is activated by raising the data set ready (DSR) signal, it functions exactly as a line configured with the modem dialin line configuration command described in the "Automatically Answer a Modem" section earlier in this chapter. If the line is activated by an incoming TCP connection, the line functions similarly to lines not used with modems.
Figure 38 EXEC and Daemon Creation for Incoming and Outgoing Calls
Note
Configure a Line Timeout Interval
You can change the interval that the Cisco IOS software waits for the CTS signal after raising the DTR signal in response to the DSR (the default is 15 seconds). To do so, perform the following task in line configuration mode. The timeout applies to the modem callin command only.
Note
Close Modem Connections
You can configure a line to close connections from a user's terminal when the terminal is turned off and prevent inbound connections to devices that are out of service. To do so, perform the following task in line configuration mode:
Figure 39 illustrates the modem cts-required command operating in the context of a continuous CTS signal. This form of modem control requires that the CTS signal be high for the entire session. If CTS is not high, the user's input is ignored and incoming connections are refused (or sent to the next line in a rotary group).
Note
Figure 39 EXEC and Daemon Creation on a Line Configured for Continuous CTS
Configure a Line to Automatically Disconnect
You can configure automatic line disconnect by performing the following task in line configuration mode:
The autohangup command causes the EXEC facility to issue the exit command when the last connection closes. This feature is useful for UNIX-to-UNIX copy program (UUCP) applications because UUCP scripts cannot issue a command to hang up the telephone. This feature is not often used.
Support Old-Style Dial-In Modems
The Cisco IOS software supports dial-in modems that use DTR to control the off-hook status of the telephone line. This feature is supported primarily on old-style modems, especially those in Europe. To configure the line to support this feature, perform the following task in line configuration mode:
Figure 40 illustrates the modem callin command. When a modem dialing line is idle, it has its DTR signal at a low state and waits for a transition to occur on the DSR (RING) input. This transition causes the line to raise the DTR signal and start watching the CTS signal from the modem. After the modem raises CTS, the Cisco IOS software creates an EXEC session on the line. If the timeout interval (set with the modem answer-timeout command) passes before the modem raises the CTS signal, the line lowers the DTR signal and returns to the idle state.
Figure 40 EXEC Creation on a Line Configured for Modem Call-In
Note
Although you can use the modem callin line configuration command with newer modems, the modem dialin line configuration command described in this section is more appropriate. The modem dialin command frees up CTS input for hardware flow control. Modern modems do not require the assertion of DTR to answer a phone line (that is, to take the line off-hook).
Support Reverse Modem Connections and Prevent Incoming Calls
In addition to initiating connections, the Cisco IOS software can receive incoming connections. This capability allows you to attach serial and parallel printers, modems, and other shared peripherals to the router or access server and drive them remotely from other modem-connected systems. The Cisco IOS software supports reverse TCP, XRemote, and LAT connections.
The specific TCP port or socket, to which you attach the device determines the type of service that the Cisco IOS software provides on a line. When you attach the serial lines of a computer system or a data terminal switch to the serial lines of the access server, the access server can act as a network front-end device for a host that does not support the TCP/IP protocols. This arrangement is sometimes called front-ending, or reverse connection mode.
The Cisco IOS software supports ports connected to computers that are connected to modems. You can configure the Cisco IOS software to function somewhat like a modem by performing the following task in line configuration mode. This command also prevents incoming calls.
Figure 41 illustrates the modem callout process. When the Cisco IOS software receives an incoming connection, it raises the DTR signal and waits to see if the CTS signal is raised to indicate that the host has noticed the router's DTR signal. If the host does not respond within the interval set by the modem answer-timeout line configuration command, the software lowers the DTR signal and drops the connection.
Figure 41 Daemon Creation on a Line Configured for Modem Call-out
Asynchronous Configuration Task List
This section describes how to configure basic functionality on asynchronous interfaces, and then customize the interfaces for your environment. Basic configuration tasks include the following:
•
•
•
•
•
•
•
•
If you want to call back a PPP client requesting asynchronous callback, refer to the chapter "Configuring Asynchronous Callback" in this publication.
See the "Asynchronous Configuration Examples" section at the end of this chapter for examples of asynchronous configuration files. Tasks are performed in global configuration mode unless otherwise specified.
Specify an Asynchronous Interface
On an access server, you can configure asynchronous interfaces. The auxiliary port (labeled AUX on the back of the product) can also be configured as an asynchronous serial interface, although performance on the AUX port is much slower than on standard asynchronous interfaces and does not support some features. illustrates why asynchronous interfaces permit substantially better performance than AUX ports configured as asynchronous interfaces.
Table 7 Differences between Auxiliary (AUX) Port and Asynchronous Port
Maximum speed
115200 kbps
38400 kbps
Supports DMA buffering1
Yes
No
PPP framing on chip2
Yes
No
IP fast switching3
Yes
No
1 Direct Memory Access (DMA) buffering moves data packets directly to and from system memory without interrupting the main CPU. This process removes overhead from the CPU and increases overall system performance.
2 PPP framing on a hardware chip removes overhead from the router's CPU, which enables the router to sustain 115.2-kbps throughput on all asynchronous ports simultaneously.
3 After the destination of the first IP packet is added to the fast switching cache, it is fast switched to and from other interfaces with minimal involvement from the main processor.
On routers without built-in asynchronous interfaces, only the AUX port can be configured as an asynchronous serial interface. To configure the AUX port as an asynchronous interface, you must also configure it as an auxiliary line with the line aux 1 command.
Use the line command with the appropriate line configuration commands for modem control, such as speed. Perform the following task in global configuration mode to specify a port as an asynchronous interface:
Configure Group and Member Asynchronous Interfaces
You can create an asynchronous interface to be used as a group interface, which can be associated with other, member asynchronous interfaces.
This association allows you to configure the group interface and all of its member interfaces with a single command entered at the asynchronous group interface command line. You can have more than one group interface on a device; however, a member interface can be associated with only one group.
See the "Group and Member Asynchronous Interfaces Examples" section later in this chapter for an example of group and member interfaces.
illustrates the group-member interface concept.
Figure 42 Group-Member Association on Asynchronous Interfaces
