📄 connecting a slipppp device to a router's aux port.htm
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<title>Cisco - Connecting a SLIP/PPP Device to a Router's AUX Port</title>
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<h1>Connecting a SLIP/PPP Device to a Router's AUX Port</h1>
<hr>
<h2>Introduction</h2>
<p>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. </p>
<h2>Physical Set Up</h2>
<p>First get the physical portion of the config working - the
modem and the wiring. </p>
<ol>
<li>Set the modem serial interface connection to the highest
speed the modem supports, up to 38400. <pre> <b> line aux 0
rxspeed 38400
txspeed 38400
</b></pre>
</li>
<li>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. <pre>
<b> modem inout
</b></pre>
</li>
<li>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. <pre>
<b> flowcontrol hardware
</b></pre>
</li>
<li>Choose not to time out when no input is received for a
time. <pre>
<b> exec-timeout 0 0
</b></pre>
</li>
<li>Give yourself a prompt when you connect to the router. <pre>
<b> exec
</b></pre>
</li>
</ol>
<p>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 <b>AT</b> and you should see an OK. After entering all
the commands, type <b>Crtl-Shft-6</b>, then <b>x</b>. This will
return you to a router prompt. Type <b>disc</b> to disconnect
your session. Now the DTE speed of your modem is set and the
connection to the modem is verified. </p>
<p>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. </p>
<p>Make sure that flowcontrol of the line is working. Issue the
command <b>term length 0</b>, then <b>show memory</b>. 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. </p>
<p>Now verify that modem control works when you issue the <b>quit</b>
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 <b>modem
inout</b> 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 <b>modem inout</b> set. </p>
<p>Of course, the modem could also be configured from an attached
terminal. This terminal must be set to 38400 to set port speed. </p>
<p>When you're sure of the physical link integrity, move on to
the SLIP/PPP configuration. </p>
<h2>Logical Set Up</h2>
<p>The next step is to get the logical portion of the config
working - the SLIP or PPP connection. </p>
<ol>
<li>Enable IP connectivity on AUX 0 (AUX 0 is async 1). <pre>
<b>interface async 1
</b></pre>
</li>
<li>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. <pre>
<b>ip unnumbered ethernet 0
</b></pre>
</li>
<li>Use TCP header compression if the connecting system uses
it. <pre>
<b>ip tcp header-compression passive
</b></pre>
</li>
<li>Default to PPP encapsulation. <pre>
<b>encapsulation PPP
</b></pre>
</li>
<li>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. <pre>
<b>async default ip address 131.108.75.2
</b></pre>
</li>
<li>Allow the user to use SLIP or PPP. <pre>
<b>async mode interactive
</b></pre>
</li>
</ol>
<p>The end device dialing in should issue the command <b>SLIP</b>
or <b>PPP</b> 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. </p>
<p>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 <b>BOOTP</b>
for SLIP or <b>IPCP</b> 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 <b>async default ip address</b>. </p>
<p>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 <b>async
default ip address</b>. If it does not, check with the package
manufacturer that the package is configured correctly. </p>
<h2>Troubleshooting</h2>
<p><b>Problem:</b> The modem doesn't answer. </p>
<p><b>Solution:</b> 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. </p>
<p><b>Problem:</b> The modem answers, but no router prompt is
returned (VT100 mode). </p>
<p><b>Solution:</b> 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. </p>
<p><b>Problem:</b> The SLIP or PPP connection is made, but you
can't telnet or ping the Ethernet interface. </p>
<p><b>Solution:</b> Check IP addresses for PC and async
interface. Use show interface to check the async interface
protocol state. </p>
<p><b>Problem:</b> You can ping or telnet to the Ethernet
interface, but nothing else. </p>
<p><b>Solution:</b> The PC doesn't have default-gateway. There
could be some other IP routing problem. </p>
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