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Introduction

This document will walk you through the process of connecting a SLIP or PPP client to a router's AUX port. There are two main steps in the process - physical set up and logical set up. The configuration shown here will work for routers running 10.0 or later versions of the Cisco IOS.

Physical Set Up

First get the physical portion of the config working - the modem and the wiring.

1. Set the modem serial interface connection to the highest speed the modem supports, up to 38400.

         line aux 0
   rxspeed 38400
   txspeed 38400
   

2. Configure the AUX port for modem inout. This means that the modem must be set to provide CD since the router will disconnect when its CD signal drops. Also, the router will drop DTR if it wants the modem to disconnect. The modem most be programmed to hang up when DTR drops.

    modem inout
   

3. Use hardware flow control (RTS/CTS). The AUX port will drop RTS when it wants the modem to disconnect, and the modem must drop CTS if it wants to flowcontrol the AUX port. The modem must be programmed for RTS/CTS.

    flowcontrol hardware
   

4. Choose not to time out when no input is received for a time.

    exec-timeout 0 0
   

5. Give yourself a prompt when you connect to the router.

    exec
   

In order to configure the modem, enter the appropriate AT commands from the fax document Basic Modem Cabling and Configuration. One way to do this is to telnet to the AUX port using the IP address of the Ethernet + 2001. For example, if the IP address of your Ethernet is 156.32.4.1, telnet to 156.32.4.1 2001. Type AT and you should see an OK. After entering all the commands, type Crtl-Shft-6, then x. This will return you to a router prompt. Type disc to disconnect your session. Now the DTE speed of your modem is set and the connection to the modem is verified.

Now dial in using an async VT100 terminal emulation. If you see NO PROMPT, verify that your wiring is correct (straight through cable for A/M/CGS, 7000, 4000 and 3000 series; 8 pin modular cable and 8 pin to 25 pin adapter for 2500 series). See the fax document Cabling Guide for RJ-45 Console and AUX Ports for more details.

Make sure that flowcontrol of the line is working. Issue the command term length 0, then show memory. You should see perfectly aligned columnar output. If the data does not line up, characters are getting dropped. Check the flowcontrol settings on your aux port and modem, and on your dial-in modem and terminal.

Now verify that modem control works when you issue the quit command at the exec prompt. If your modem loses carrier, the DTR portion of the modem control is set properly. If the modem does not hang up, check the modem on the aux port to make sure it is set to hang up on loss of DTR. Also make sure the aux port has modem inout set. In order to test the CD portion of the modem control, enter the enable mode and then force your local modem to hang up. When you reconnect, you should not be in enable mode. If you are, the aux port is not recognizing the loss of carrier. Check the wiring, the modem settings, and make sure the aux port has modem inout set.

Of course, the modem could also be configured from an attached terminal. This terminal must be set to 38400 to set port speed.

When you're sure of the physical link integrity, move on to the SLIP/PPP configuration.

Logical Set Up

The next step is to get the logical portion of the config working - the SLIP or PPP connection.

1. Enable IP connectivity on AUX 0 (AUX 0 is async 1).

   interface async 1
   

2. Set the address of the aux port to the local Ethernet port. This allows the end device to have a phantom presence on the Ethernet.

   ip unnumbered ethernet 0
   

3. Use TCP header compression if the connecting system uses it.

   ip tcp header-compression passive
   

4. Default to PPP encapsulation.

   encapsulation PPP
   

5. Set a default IP address in case the user doesn't specify one when dialing in. Use the same subnet as the unnumbered reference (int E 0). This is the address of the node that is calling in.

   async default ip address 131.108.75.2
   

6. Allow the user to use SLIP or PPP.

   async mode interactive
   

The end device dialing in should issue the command SLIP or PPP to start up SLIP or PPP services. Most packages have a scripting language that will allow you to issue text commands before going in to SLIP or PPP mode.

When the command is issued, the router reports a text message containing the IP address it expects the remote end to have. One way you can set the address of the node calling in is to manually read that address and program it in. Some packages automatically read that message. A better way to set this address is to use BOOTP for SLIP or IPCP for PPP. This must be configured on the dial-in client. If you use IPCP with PPP, you can set the end node address to 0.0.0.0 and then it will dynamically learn the address that you have set using async default ip address.

To test connectivity, ping the address of the Ethernet port. If that works, start pinging some hosts. If this works, your connectivity is fine. If it doesn't work, the address may be set wrong on your end host, or you may have a noisy connection, or you may have poor physical connectivity. Make sure you have passed all the steps in part 1. Make sure that the package you are using knows the address that you programmed with async default ip address. If it does not, check with the package manufacturer that the package is configured correctly.

Troubleshooting

Problem: The modem doesn't answer.

Solution: Set the S0 register (ATS0=1) or the auto-answer dipswitch (if present). DTR may not be present at the modem because of a modem inout or cable problem.

Problem: The modem answers, but no router prompt is returned (VT100 mode).

Solution: Check the DTE port speeds of the modem, router, and PC. Also verify that modem inout is set and that the router recognizes an active session on the AUX port.

Problem: The SLIP or PPP connection is made, but you can't telnet or ping the Ethernet interface.

Solution: Check IP addresses for PC and async interface. Use show interface to check the async interface protocol state.

Problem: You can ping or telnet to the Ethernet interface, but nothing else.

Solution: The PC doesn't have default-gateway. There could be some other IP routing problem.

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