Table Of Contents
Establishing Telnet Sessions
Establishing a Reverse Telnet Connection
Troubleshooting the Telnet Connection
Testing the Modem Connection
Suspending and Terminating Telnet Sessions
Using UDP Telnet to Connect
Asynchronous Serial Traffic over UDP Feature Overview
Asynchronous Serial Traffic over UDP Configuration Task List
Preparing to Configure Asynchronous Serial Traffic over UDP
Configuring a Line for UDPTN
Enabling UDPTN
Verifying UDPTN Traffic
UDPTN Configuration Examples
Multicast UDPTN
Broadcast UDPTN
Point-to-Point UDPTN
Using Cisco DialOut for Telnet Connections
Establishing Telnet Sessions
This chapter describes how to communicate with a modem by establishing Telnet sessions from the asynchronous line on an access server or router to the modem. It includes the following main sections:
•
Establishing a Reverse Telnet Connection
•Troubleshooting the Telnet Connection
•Testing the Modem Connection
•Suspending and Terminating Telnet Sessions
•Using UDP Telnet to Connect
•Using Cisco DialOut for Telnet Connections
For a complete description of the commands mentioned 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.
Establishing a Reverse Telnet Connection
A reverse Telnet session is sometimes referred to as direct Telnet. When you reverse Telnet you are initiating a Telnet session out the asynchronous line, instead of accepting a connection into the line (which is a forward connection). To establish a reverse Telnet session to a 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 2001
Trying 172.16.1.10, 2001 ... Open
This 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 reverse Telnet sessions with modems. The ip host command maps an IP address of a port to a device name.
Troubleshooting the Telnet Connection
If you are unable to connect to the modem, check the following:
Step 1 Enter the show users EXEC command. It should not indicate the line is in use.
Step 2 Scan the show users command output to verify that the line is configured for modem inout.
Step 3 Enter the show line EXEC command. The output should contain the following two lines:
Modem hardware state: CTS noDSR DTR RTS
Step 4 Check whether the virtual terminal connections to lines in the access server require passwords. See the Cisco IOS Security Configuration Guide for additional information about assigning passwords to virtual terminals.
Step 5 Check whether the speed between the modem and the access server are the same. It is likely to be different. If so, switch off the modem, then switch it back on. The speed of the modem should now match the speed of the access server.
Testing the Modem Connection
After you make a reverse 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:
If the modem does not reply to the at command, check the following:
Step 1 Enter the show line EXEC command (see Step 3 in the previous section). Look at the output of the command. If it displays "no CTS" for the modem hardware state, the modem is not connected or powered on, and is waiting for data; or the modem might not be configured for hardware flow control.
Step 2 Check your cabling and the modem configuration (echo or result codes might be off). Enter the appropriate AT modem command to view the modem configuration, or enter the at&f command to return to factory defaults. Refer to your modem documentation to learn the appropriate AT command to view your modem configuration.
Suspending and Terminating Telnet Sessions
The reverse 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.
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 Ensure that you can reliably enter the escape sequence to suspend a Telnet session. Some terminal emulator packages have difficulty sending the Ctrl-Shift-6 x sequence. Refer to your terminal emulator documentation for more information about escape sequences.
To suspend and then disconnect a Telnet session, perform the following steps:
Step 1 Suspend the Telnet session by entering Ctrl-Shift-6 x:
Step 2 Enter the where EXEC command to check for open sessions:
Conn Host Address Byte Idle Conn Name
* 1 172.16.1.10 172.16.1.10 0 0 172.16.1.10
2 172.16.1.11 172.16.1.11 0 12 modem2
Step 3 After suspending a session with one modem, connect to another modem, then suspend it, as follows:
Trying modem2 (172.16.1.11, 2002) ... Open
Step 4 To disconnect (completely close) a session, enter the disconnect EXEC command:
router# disconnect line 1
Closing connection to 172.16.1.10 [confirm] y
router# disconnect line 2
Closing connection to 172.16.1.11 [confirm] y
Note Before attempting to allow inbound connections, make sure you close all open connections to the modems attached to the access server. If you have a modem port in use, the modem will not accept a call properly.
Using UDP Telnet to Connect
The Asynchronous Serial Traffic over UDP feature provides the ability to encapsulate asynchronous data into User Datagram Protocol (UDP) packets, and then unreliably send this data without needing to establish a connection with a receiving device. This process is referred to as UDP Telnet (UDPTN), although it does not—and cannot—use the Telnet protocol. UDPTN is similar to Telnet in that both are used to send data, but UDPTN is unique in that it does not require that a connection be established with a receiving device.
You load the data you want to send through an asynchronous port, and then send it, optionally, as a multicast or a broadcast. The receiving device(s) can then receive the data whenever it wants. If the receiver ends reception, the transmission is unaffected.
Asynchronous Serial Traffic over UDP Feature Overview
The Asynchronous Serial Traffic over UDP feature provides a low bandwidth, low maintenance method to unreliably deliver data. This delivery is similar to a radio broadcast: It does not require that you establish a connection to a destination; rather, it sends the data to whatever device wants to receive it. The receivers are free to begin or end their reception without interrupting the transmission.
This feature is particularly useful for broadcast, multicast, and unstable point-to-point connections.
It is a low-bandwidth solution for delivering streaming information for which lost packets are not critical. Such applications include stock quotes, news wires, console monitoring, and multiuser chat features.
This feature may not work as expected when there are multiple users on the same port number in a nonmulticast environment. The same port must be used for both receiving and sending.
Asynchronous Serial Traffic over UDP Configuration Task List
To configure the Asynchronous Serial Traffic over UDP feature, perform the tasks described in the following sections:
•Preparing to Configure Asynchronous Serial Traffic over UDP (Required)
•Configuring a Line for UDPTN (Required)
•Enabling UDPTN (Required)
•Verifying UDPTN Traffic (Optional but Recommended)
See the "UDPTN Configuration Examples" section at the end of this chapter for multicast, broadcast, and point-to-point UDPTN configuration examples.
Preparing to Configure Asynchronous Serial Traffic over UDP
When configuring the Asynchronous Serial Traffic over UDP feature for multicast transmission, you must configure IP multicast routing for the entire network that will receive or propagate the multicasts. When configuring the feature for broadcast transmission, you must configure broadcast flooding on the routers between network segments. See the "Configuring IP Multicast Routing" chapter of this guide for information on how to configure IP multicast routing. See the section "Configuring Broadcast Packet Handling" in the Cisco IOS IP and IP Routing Configuration Guide for information on how to configure broadcast flooding.
Configuring a Line for UDPTN
To configure the line that will be used to send or receive UDP packets, use the following commands beginning in global configuration mode:
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Command
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Purpose
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Step 1
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Router(config)# line line-number
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Enters line configuration mode.
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Step 2
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Router(config-line)# transport output udptn
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Enables the line to transport UDP packets.
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Step 3
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Router(config-line)# dispatch-timeout 1000
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Sends packets every 1000 milliseconds.
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Step 4
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Router(config-line)# dispatch-character 13
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Sends packets after every new line.
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Step 5
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Router(config-line)# no session-timeout
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Disables timeout connection closing.
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Enabling UDPTN
There are two methods of enabling UDPTN. You can either manually enable UDPTN when you want to begin transmission or reception, or you can configure the router to automatically enable UDPTN when a connection is made to the line.
To manually enable UDPTN and begin UDPTN transmission or reception, enter the following EXEC command at the system prompt:
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Command
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Purpose
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Router# udptn ip-address [port] [/transmit] [/receive]
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Enables UDPTN to the specified IP address (optionally, using the specified port). Use the /transmit or /receive keywords if the router will only be sending or receiving UDPTN.
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To automatically enable UDPTN when a connection is made to the line, use the following line configuration commands beginning in global configuration mode:
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Command
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Purpose
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Step 1
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Router(config)# line line-number
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Enters line configuration mode.
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Step 2
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Router(config-line)# autocommand udptn ip-address [port] [/transmit] [/receive]
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Enables UDPTN automatically when a connection is made to the line (optionally, using the specified port). Use the /transmit or /receive keywords if the router will only be sending or receiving UDPTN.
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Verifying UDPTN Traffic
To verify UDPTN is enabled correctly, perform the following steps:
Step 1 Enable UDPTN debugging by using the debug udptn EXEC command.
Step 2 Enable UDPTN by using the udptn ip-address EXEC command.
Step 3 Observe the debug output.
The following debug output shows a UDPTN session being successfully established and then disconnected.
Trying 172.16.1.1 ... Open
*Mar 1 00:10:15.191:udptn0:adding multicast group.
*Mar 1 00:10:15.195:udptn0:open to 172.16.1.1:57 Loopback0jjaassdd
*Mar 1 00:10:18.083:udptn0:output packet w 1 bytes
*Mar 1 00:10:18.087:udptn0:Input packet w 1 bytes
Closing connection to 172.16.1.1 [confirm] y
*Mar 1 00:11:03.139:udptn0:removing multicast group.
Step 4 While the udptn command is enabled, enter the show ip socket command to verify that the socket being used for UDPTN opened correctly.
Proto Remote Port Local Port In Out Stat TTY OutputIF
17 --listen-- 172.21.14.90 67 0 0 89 0
17 0.0.0.0 520 172.21.14.90 520 0 0 1 0
17 1.1.1.2 57 1.1.1.1 57 0 0 48 0
17 224.1.1.1 57 1.2.2.2 57 0 0 48 0 Loopback0
UDPTN Configuration Examples
This section provides the following UDPTN configuration examples:
•Multicast UDPTN
•Broadcast UDPTN
•Point-to-Point UDPTN
Multicast UDPTN
These configurations are for multicast UDPTN. The router that is multicasting does not require a multicast configuration—it simply sends to the multicast IP address.
Router That Is Multicasting
ip address 10.1.1.1 255.255.255.0
autocommand udptn 172.1.1.1 /transmit
Receiving Routers
ip address 10.99.98.97 255.255.255.192
transport output udptn telnet lat rlogin
autocommand udptn 172.1.1.1 /receive
Broadcast UDPTN
These configurations are for broadcast UDPTN. This is the simplest method to send to multiple receivers. The broadcasting router sends to the broadcast IP address, and any router that wants to receive the transmission simply connects to the broadcast IP address by using the udptn command.
Router That Is Broadcasting
ip address 10.1.1.1 255.255.255.0
autocommand udptn 255.255.255.255 /transmit
Receiving Routers
ip address 10.99.98.97 255.255.255.192
transport output udptn telnet lat rlogin
autocommand udptn 255.255.255.255 /receive
Point-to-Point UDPTN
These configurations are for two routers in mobile, unstable environments that wish to establish a bidirectional asynchronous tunnel. Because there is no way to ensure that both routers will be up and running when one of the routers wants to establish a tunnel, they cannot use connection-dependent protocols like Telnet or LAT. They instead use the following UDPTN configurations.
Notice that each router is configured to send to and receive from the IP address of the other. Because both routers will be sending and receiving, they do not use the /transmit or /receive keywords with the udptn command.
Router A
ip address 10.54.46.1 255.255.255.192
autocommand udptn 10.54.46.2
Router B
ip address 10.54.46.2 255.255.255.192
autocommand udptn 10.54.46.1
Using Cisco DialOut for Telnet Connections
The Cisco DialOut feature enables users on a workstation operating Windows to send faxes or connect to service provider services outside the LAN by using modems attached or internal to a network access server. The Cisco DialOut feature extends the functionality of Telnet by enabling users to control the activity of these modems from their desktop computers using standard communications software.
The Cisco DialOut feature has two components:
•Telnet Extensions for Dialout—Network access server component
•The DialOut Utility—Client/desktop component
Both components are required and neither can function as a stand-alone feature.
The Telnet Extensions for Dialout component uses reverse Telnet to access modems attached to the network access server. This component enables the network access server to interface with the client/desktop component of the Cisco DialOut feature and to return Carrier Detect signals to the communications software so that the software can determine when to start dialing a particular number.
Telnet extensions allow the communications software running on the desktop computer of the client to control modem settings such as baud rate, parity, bit size, and stop bits.
To enable this feature, you only need to configure the access server or router for reverse Telnet and configure the appropriate lines to send and receive calls.
The client/desktop component of Cisco DialOut feature must be installed on the client workstation before this feature can be used. For information about installing and using the client/desktop component of the Cisco Dial-Out feature, and configuring the access server, see the DialOut Utility User Guide Cisco publication in CCO.
