Click on the routers in the topology below to view their configurations.

router 2500 AUX----NULL MODEN CABLE-----ROUTER 2500 AUX

Before You Begin

This configuration has Cisco 2500-series routers back-to-back using the async AUX port. The Cisco AUX port is connected to another Cisco AUX port running PPP. The connection has been configured to allow IP for a permanent connection.

Ingredients

For this recipe, you need the following:

A null modem cable.

You may also need an RJ45-to-DB25 adapter converter. For more information on cables, see the Cables documentation.

Router 1

version 11.1

service udp-small-servers

service tcp-small-servers

!

hostname Router1

!

interface Ethernet0

no ip address

shutdown

!

interface Serial0

no ip address

shutdown

!

interface Serial1

no ip address

shutdown

!

interface Async1

ip address 192.168.10.1 255.255.255.0

! -- the IP address on the AUX ports of both routers must be in the same subnet

encapsulation ppp

! -- set PPP as encapsulation

async dynamic routing

async mode dedicated

!

no ip classless

ip route 0.0.0.0 0.0.0.0 Async1

logging buffered

!

line con 0

exec-timeout 0 0

line aux 0

modem InOut

transport input all

rxspeed 38400

txspeed 38400

flowcontrol hardware

line vty 0 4

login

!

end

Router 2

version 11.1

service udp-small-servers

service tcp-small-servers

!

hostname Router2

!

!

interface Ethernet0

no ip address

shutdown

!

interface Serial0

no ip address

shutdown

!

interface Serial1

no ip address

shutdown

!

interface Async1

ip address 192.168.10.2 255.255.255.0

! -- the IP address on the AUX ports of both routers must be in the same subnet

encapsulation ppp

async dynamic routing

async mode dedicated

!

no ip classless

ip route 0.0.0.0 0.0.0.0 Async1

logging buffered

!

line con 0

exec-timeout 0 0

line aux 0

modem InOut

transport input all

rxspeed 38400

txspeed 38400

flowcontrol hardware

line vty 0 4

login

!

end

Debug and Show Commands to Use for Troubleshooting

Check under line aux 0:

Issue the show line aux 0 command.

Verify that all signals are up. For example, CTS, RTS, DTR, CD. If nothing appears next to them, they are up. If "No" appears next to them (for example, No-CTS), it means they are down.

Router1#show line aux 0

Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns

A 1 AUX 38400/38400 - inout - - - 0 0 0/0

Line 1, Location: "", Type: ""

Length: 24 lines, Width: 80 columns

Baud rate (TX/RX) is 38400/38400, no parity, 2 stopbits, 8 databits

Status: Ready, Active, Async Interface Active

Capabilities: Hardware Flowcontrol In, Hardware Flowcontrol Out

Modem Callout, Modem RI is CD, Line is permanent async interface

Modem state: Ready

Special Chars: Escape Hold Stop Start Disconnect Activation

^^x none - - none

Timeouts: Idle EXEC Idle Session Modem Answer Session Dispatch

00:10:00 never none not set

Modem type is unknown.

Session limit is not set.

Time since activation: 00:00:30

Editing is enabled.

History is enabled, history size is 10.

Full user help is disabled

Allowed transports are pad v120 telnet. Preferred is telnet.

No output characters are padded

No special data dispatching characters

Line is running PPP for address 192.168.10.2.

0 output packets queued, 1 input packets.

Async Escape map is 00000000000000000101000000000000

Modem hardware state: CTS* DSR* DTR RTS

Use the show line command to determine the async interface of the AUX ports. Though most routers have the AUX port as interface async 1, Access servers have the AUX port interface after the tty lines. For example, if your router has 16 async/modem lines, then the AUX port is line 17. In this case, the AUX port must be configured on interface async 17. Configure the AUX port based on the show line outputs. The example below verifies that the AUX port is on int async 1.

Router1#show line

Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns

* 0 CTY - - - - - 0 0 0/0

A 1 AUX 38400/38400 - inout - - - 0 0 0/0

2 VTY - - - - - 0 0 0/0

3 VTY - - - - - 0 0 0/0

4 VTY - - - - - 0 0 0/0

5 VTY - - - - - 0 0 0/0

6 VTY - - - - - 0 0 0/0

Use these debug commands to verify that the link comes up correctly:

debug ppp negotiation - To display information on PPP traffic and exchanges while negotiating Link Control Protocol (LCP), Authentication, and Network Control Protocol (NCP). A successful PPP negotiation will first open the LCP state, then Authenticate, and finally negotiate NCP. If CONFREQs are being sent out, but no CONFACKs being received, check that the cabling is correct and whether the correct line (interface async x) is configured.

debug ppp authentication - To display PPP authentication protocol messages, including Challenge Handshake Authentication Protocol (CHAP) packet exchanges and Password Authentication Protocol (PAP) exchanges.

debug ppp error - To display protocol errors and error statistics associated with PPP connection negotiation and operation.

Before attempting any of the above debug commands, please see Important Information on Debug Commands

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