Digi 34 Application Note

1

Application Note 34

Configuring ADSL to backup to PSTN/ISDN with

automatic testing and recovery back to ADSL.

UK Support

29 December 2009

2

Contents

1 Introduction ............................................................................................................................................... 2

1.1 Outline ............................................................................................................................................... 2

1.2 Assumptions ...................................................................................................................................... 2

1.3 Corrections ........................................................................................................................................ 3

1.4 Version ............................................................................................................................................... 3

2 Configuration ............................................................................................................................................. 4

2.1 Configure PPP 1 (ADSL) ...................................................................................................................... 4

2.2 Configure PPP 4 (PSTN) ...................................................................................................................... 6

2.3 Configure the Default Routes ............................................................................................................ 8

2.4 Configure the Firewall ....................................................................................................................... 9

2.5 Save configuration ........................................................................................................................... 10

3 Testing ..................................................................................................................................................... 11

3.1 Simulate a failure ............................................................................................................................. 11

4 Configuration Files ................................................................................................................................... 11

4.1 Digi Transport Configuration Files ................................................................................................... 11

4.2 Digi Transport Firmware Versions ................................................................................................... 12

1 INTRODUCTION

1.1 Outline

The document describes how to configure a Digi Tran sport router to fail over to PSTN when there is a
problem with the main ADSL connection. Once the ADSL connection is working again the router will test it
by sending pings and then revert back to ADSL and drop the PSTN connection.
This application note uses PPP 4 (PSTN) as the failov er i nterface, however, if the router being configured
has ISDN, simply configure PPP 2 (ISDN) instead of PPP 4. The same configuration can be applied to the
ISDN interface.

For failover from ADSL to a cellular network, please see Application Note 34a as the configuration is slightly
different.

1.2 Assumptions

This guide has been written for use by technically competent personnel with a good understanding of the
communications technologies used in the product, and of the requirements for their specific application.
Configuration: This application note assumes that the router will be connecting to a cellular network.

This application note applies to;

Models shown: Digi Transport DR64 router with ADSL & PSTN running firmware version 5081.
Other Compatible Models: All Digi Transport products.

Firmware versions: 4905 or later.

3

Configuration: This Application Note assumes that th e Digi Transport router is already configured with a
working ADSL connection through PPP 1.

1.3 Corrections

Requests for corrections or amendments to this application note are welcome and should be addressed t o:

[e-mail]

Requests for new application notes can be sent to the same address.

1.4 Version

Version Number Status

1.0

Published

1.1

Re-branded to Digi Transport

4

2 CONFIGURATION

2.1 Configure PPP 1 (ADSL)

This PPP interface should already be configured f or use with the ISP. You should confirm that you have a
working ADSL connection before proceeding with this configuration.
The PPP 1 interface needs to be configured to be “always on”.
The inactivity timer should be set to 0.

Configuration - Interfaces > PPP > PPP 0 - 4 > PPP 1 > Standard

Parameter Setting Description

Always on mode ON Configures interface as always on

Inactivity timeout (s): 0 Interface is always connected and active

Click OK
PPP 1 should be configured to generate ICMP requests t hat the firewall can monitor and use to detect when
the ADSL connection has a problem.
It is also necessary to configure a settling tim e on the interface. The settling time is a parameter that
prevents the Digi Transport from using the interface until it has been up for the specified number of seconds.
This delay is required so that any ICMP echo request s (used for testing the interface) are not sent so soon
after the interface has come up that the ISP drops the reply packet. (ISP routing tables are often not updated
within the first few seconds of an interface coming up. Sending packets immediately often causes the reply
to these packets to be dropped.)

Configuration - Interfaces > PPP > PPP 0 - 4 > PPP 1 > Advanced

Parameter Setting Description

Ping request interval 10 Send ICMP requests every x seconds

Only send PINGs when

interface is in service

YES Do not send ICMP requests if using cellular

PING IP address 1.2.3.4

A public IP address that you can ping to

check connectivity via ADSL

Settling time (*100ms) 100

Do not send 1st ping until 10 seconds after

interface has come up