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This chapter describes how to configure the Cisco IOS software to call back an asynchronous device that dials in and requests a callback from the router, then disconnects. It includes the following main sections:
For a complete description of the commands in this chapter, see the Cisco IOS Dial Services Command Reference publication. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
Asynchronous callback is supported for the following protocols:
Callback is also supported on other interface types for PPP, including ISDN.
All callback sessions are returned on physical terminal (TTY) lines. ARA is supported on virtual terminal lines, but also is supported on TTY lines if the vty-arap command is used. PPP, however, is supported on interfaces. Therefore, to enable PPP callback, you must enter the autoselect ppp command on the callback lines.
All current security mechanisms supported in the Cisco IOS software are supported by the callback facility, including the following:
The call originator must have the appropriate permissions set on the router before it can initiate a callback session.
Callback is useful for two purposes:
To configure asynchronous callback, perform the tasks in the following sections:
See the section "Asynchronous Callback Examples" at the end of this chapter for ideas on how to implement asynchronous callback.
You can call back PPP clients that dial in to asynchronous interfaces. You can enable callback to the following two types of PPP clients:
This section describes how to enable callback to each of these types of PPP clients.
To accept a callback request from an RFC 1570 PPP-compliant client, use the following command in interface (asynchronous) configuration mode:
| Command | Purpose |
|---|---|
ppp callback accept | Enables callback requests from RFC 1570 PPP-compliant clients on an asynchronous interface. |
To configure the Cisco IOS software to call back the originating PPP client, see the section "Enabling PPP Callback on Outgoing Lines" later in this chapter.
A PPP client can put itself in answer-mode and can still be called back by the router, even though it cannot specifically request callback. To enable callback on the router to this type of client, use the following command in interface (asynchronous) configuration mode:
| Command | Purpose |
|---|---|
ppp callback initiate | Initiates callback requests from non-RFC 1570 PPP-compliant clients on an asynchronous interface. |
To configure the Cisco IOS software to call back the originating PPP client, see the next section, "Enabling PPP Callback on Outgoing Lines."
After enabling PPP clients to connect to an asynchronous interface and wait for a callback, you must place one or more TTY lines in PPP mode. Although calls from PPP clients enter through an asynchronous interface, the calls exit the client on a line placed in PPP mode.
To enable PPP client callback on outgoing TTY lines, use the following commands beginning in global configuration mode:
| Command | Purpose | |
|---|---|---|
Step1 | chat-script script-name expect-send | Defines a chat script to be applied when a PPP client requests callback. |
Step2 | username name [callback-dialstring telephone-number] | Specifies a per-username callback dial string. |
Step3 | username name [callback-rotary rotary-group-number] | Specifies a per-username rotary group for callback. |
Step4 | username name [callback-line [tty] line-number [ending-line-number]] | Specifies a per-username line or set of lines for callback. |
Step5 | line [tty] line-number [ending-line-number] | Enters line configuration mode. |
Step6 | autoselect ppp | Configures automatic PPP startup on a line or set of lines. |
Step7 | login {authentication | local}
| Enables authentication on the line. |
Step8 | script callback regexp | Applies a chat script to a line or set of lines. |
Step9 | callback forced-wait number-of-seconds | Delays the callback for client modems that require a rest period before receiving a callback. |
A client can issue a callback dial string; that dial string is used only if the dial string on the router is specified as NULL, or is not defined. The recommended PPP chat script follows:
chat-script name ABORT ERROR ABORT BUSY "" "ATZ" OK "ATDT \T" TIMEOUT 30 CONNECT \c
See the section "Callback to a PPP Client" at the end of this chapter for a configuration example.
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NoteNormally a router avoids line and modem noise by clearing the initial data received within the first one or two seconds. However, when the autoselect PPP feature is configured, the router flushes characters initially received and then waits for more traffic. This flush causes time out problems with applications that send only one carriage return. To ensure that the input data sent by a modem or other asynchronous device is not lost after line activation, enter the flush-at-activation line configuration command. |
You can call back clients that dial in to a TTY line and connect to the EXEC prompt. To enable callback, use the following commands beginning in global configuration mode:
| Command | Purpose | |
|---|---|---|
Step1 | service exec-callback | Enables EXEC callback. |
Step2 | chat-script script-name expect-send | Defines a chat script to be applied when clients dial in to the EXEC prompt. |
Step3 | username name [callback-dialstring telephone-number] | Specifies a per-username callback dial string. |
Step4 | username name [callback-rotary rotary-group-number] | Specifies a per-username rotary group for callback. |
Step5 | username name [callback-line [aux | tty] line-number [ending-line-number]] | Specifies a per-username line or set of lines for callback. |
Step6 | username name [nocallback-verify] | Does not require authentication on EXEC callback. |
Step7 | line [tty] line-number [ending-line-number] | Enters line configuration mode. |
Step8 | script callback regexp | |
Step9 | callback forced-wait number-of-seconds | Delays the callback for client modems that require a rest period before receiving a callback. |
The recommended EXEC chat script is as follows:
chat-script name ABORT ERROR ABORT BUSY "" "ATZ" OK "ATDT \T" TIMEOUT 30 CONNECT \c
See the section "Callback to an ARA Client" at the end of this chapter for a configuration example.
To configure callback of ARA clients, use the following commands beginning in global configuration mode. These steps assume you have already enabled AppleTalk routing and enabled ARA.
| Command | Purpose | |
|---|---|---|
Step1 | arap callback | Enables callback to an ARA client. |
Step2 | chat-script script-name expect-send | Defines a chat script to be applied when an ARA client connects to a TTY line and requests callback. |
Step3 | line [tty] line-number [ending-line-number] | Enters line configuration mode. |
Step4 | arap enable | Enables ARA on the line. |
Step5 | autoselect arap | Configures automatic protocol startup on the line. |
Step6 | login {authentication | local}
| Enables authentication on the line. |
Step7 | script arap-callback regexp | Applies an ARA-specific chat script to a line or set of lines. |
Step8 | callback forced-wait number-of-seconds | Delays the callback for client modems that require a rest period before receiving a callback. |
Step9 | exit | Exits to global configuration mode. |
Step10 | username name [callback-dialstring telephone-number] | Specifies a per-username callback dial string. |
Step11 | username name [callback-rotary rotary-group-number] | Specifies a per-username rotary group for callback. |
Step12 | username name [callback-line [tty] line-number [ending-line-number]] | Specifies a per-username line or set of lines for callback. |
The recommended ARA chat script follows and includes vendor-specific extensions on the Telebit 3000 modem to disable error control. Refer to the manual for your modem for the specific commands to disable error correction for ARA.
chat-script name ABORT ERROR ABORT BUSY "" "ATZ" OK "ATS180=0" OK "ATS181=1" OK "ATDT \T" TIMEOUT 60 CONNECT \c
See the section "Callback to a PPP Client" at the end of this chapter for an example of calling back a PPP client.
The following sections provide examples for callback:
The following example shows the process of configuring callback to a PPP client on rotary77. PAP authentication is enabled for PPP on the asynchronous interfaces. The login local command enables local username authentication on lines7, 8, and9. The remote PPP client host name is Ted, and the callback number is fixed at 1234567.
username Ted callback-dialstring "1234567" callback-rotary77
password Rhoda interface async7 ip unnumbered ethernet0 encapsulation ppp no keepalive async default ip address 1.1.1.1 async mode interactive ppp callback accept ppp authentication pap interface async8 ip unnumbered ethernet0 encapsulation ppp no keepalive async default ip address 1.1.1.2 async mode interactive ppp callback accept ppp authentication pap interface async9 ip unnumbered ethernet0 encapsulation ppp no keepalive async default ip address 1.1.1.3 async mode interactive ppp callback accept ppp authentication pap line 7 login local modem InOut rotary 77 autoselect ppp line 8 login local modem InOut rotary 77 autoselect ppp line 9 login local modem InOut rotary 77 autoselect ppp
The following example shows the process to configure an outgoing callback on the same line as the incoming request. The login local command enables local username authentication on lines4 and7. Reauthentication is required upon reconnection.
service exec-callback username milarepa callback-dialstring "" password letmein line 4 login local line 7 login local
The following example shows the process of configuring callback to an ARA client on line7. The login local command enables local username authentication on lines4 and7. Line7 will always be used for ARA callback, whether the incoming call enters line 4, 7, or 8.
appletalk routing arap callback arap network 422 router test username excalibur callback-dialstring "123456" callback-line7 password guenivere line 4 login local modem InOut autoselect arap arap enable line 7 login local modem InOut autoselect arap arap enable line 8 login local modem InOut autoselect arap arap enable
Posted: Tue Jul 18 15:16:19 PDT 2000
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