This document describes how to connect a SLIP or PPP client to the AUX
port of a router.
There are no specific requirements for this document.
The information in this document is based on the Cisco IOS® Software
Release10.0, and later versions.
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
For more information on document conventions, refer to the
Technical Tips Conventions.
You must follow two main steps when you connect a SLIP or PPP client to
the AUX port of a router:
This document explains both the steps.
The physical portion of the configuration includes the modem and the
To set up the physical portion of the configuration, complete these
Set the modem serial interface connection to the highest speed that
the modem supports. For instance, 38400.
line aux 0
Configure the AUX port for modem inout. This means that you must
set the modem to provide CD, because the router disconnects when its CD signal
drops. Also, the router drops the Data Terminal Ready (DTR) if it wants the
modem to disconnect. Program the modem to hang up when DTR drops.
Use hardware flow control (RTS/CTS). The AUX port drops Request To
Send (RTS) when it wants the modem to disconnect, and the modem must drop Clear
To Send (CTS) if it wants flowcontrol on the AUX port. Program the modem for
Specify no time out when no input is received for a time.
exec-timeout 0 0
Give yourself a prompt when you connect to the router.
In order to configure the modem, enter the appropriate AT commands from
the fax document
Connection Guide. One way to do this is to issue the
telnet command to the AUX port using the IP address
of the Ethernet + 2001. For example, if the IP address of your Ethernet is
18.104.22.168, issue telnet to 22.214.171.124 2001. Type
AT, and you should see an OK. After you enter 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.
Use an async VT100 terminal emulation to dial in. If you see NO PROMPT,
verify whether 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
Guide for RJ-45 Console and AUX Ports for details.
Ensure that flowcontrol of the line is working. Issue the command
term length 0, followed by show
memory. You should see perfectly aligned columnar output. If the
data does not line up, the implication is that characters are being dropped.
Check the flowcontrol settings on your AUX port and modem, and on your dial-in
modem and terminal.
Now verify whether 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 ensure that you have set modem inout on the AUX port. In order to
test the CD portion of the modem control, enter into the enable mode, and force
your local modem to hang up. When you reconnect, you must not be in enable
mode. If you are back in the enable mode, the AUX port does not recognize the
loss of carrier. Check the wiring and modem settings, and ensure that you have
set modem inout on the AUX port.
Of course, the modem could also be configured from an attached
terminal. Set this terminal to 38400 in order to set the port speed.
When you are sure of the physical link integrity, move on to the
The logical portion of the configuration includes the SLIP or PPP
To configure SLIP/PPP, complete these steps:
Enable IP connectivity on AUX 0. Issue the show
line command to see the AUX port tty numbering for your router.
The numbering varies based on the router model and modules installed.
Ensure that the customer knows how to identify the async port
numbering for any router.
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int
* 0 CTY - - - - - 0 0 0/0 -
I 4 AUX 9600/9600 - - - - - 0 0 0/0 -<==!!
5 VTY - - - - - 0 0 0/0 -
6 VTY - - - - - 0 0 0/0 -
7 VTY - - - - - 0 0 0/0 -
8 VTY - - - - - 0 0 0/0 -
9 VTY - - - - - 0 0 0/0 -
Line(s) 1-3 are not in async mode, or do not have hardware support.
You must configure int async 4 to configure your AUX port.
interface async 4
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
Use TCP header compression if the connecting system uses it.
ip tcp header-compression passive
Return to default PPP encapsulation.
Set a default IP address in case the user does not specify one when
it dials in. Use the same subnet as the unnumbered reference (int E 0). This is
the address of the node that is calling in.
peer default ip address 126.96.36.199
Allow the user to use SLIP or PPP.
async mode interactive
The end device that dials 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 into SLIP or PPP mode.
When you have issued the command, the router reports a text message
that contains the IP address that 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. Then it will dynamically learn the address that you have set through
async default ip address.
To test connectivity, ping the address of the Ethernet port. If that
works, start to ping some hosts. If this works, your connectivity is fine. If
it does not work, the address may be set wrongly on your end host. The other
possibilities are that you have a noisy connection or poor physical
connectivity. Ensure that you have performed all the steps in part 1. Also
enure that the package you use knows the address that you programmed with async
default IP address. If it does not, check with the package manufacturer whether
the package is configured correctly.
This section provides tips to troubleshoot some common problems.
Problem: The modem does not answer.
Solution:Set the S0 register (ATS0=1), or the
auto-answer dipswitch (if it is present). DTR may not be present at the modem
because of a modem inout or cable problem.
Problem: The modem answers, but does not return a
router prompt (VT100 mode).
Solution: Check the DTE port speeds of the modem,
router, and PC. Also verify whether 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 the
telnet or ping command
cannot be issued to the Ethernet interface.
Solution: Check IP addresses for PC and async
interface. Use the show interface command to check
the async interface protocol state.
Problem: The ping
or telnet command can be issued to the Ethernet
interface, but nothing else is possible.
Solution: The PC does not have default-gateway. There
could be some other IP routing problem.