Installing and Administering PPP

Chapter 4 79
Common pppd Options
Unique PPP Implementations
The Dialers file is ignored when “Direct” is found in the dialer field of
the Devices field. See the discussion below regarding line failovers and
using an auto-dial modem as a backup link.
Automatic Failover
The Automatic Failover option is a dial-up backup that maintains
connectivity so that IP traffic can continue when a synchronous or
dedicated asynchronous connection is dropped. User services continue
even if the dedicated line becomes unavailable, although the user may
notice the link is slower.
Setting up Automatic Failover
To set up the dial-up connection, add an entry referencing a dialup
modem after the entry for the dedicated link in the Systems file. The
added line might look like the following:
remote Any ACU 19200 5551212 in:--in: pppbackup word: password
The remote hostname must match the remote hostname entered in the
Autostart file entry described earlier in the section “Dedicated Lines.”
You also need an entry in the Devices file that accesses a device the
modem is attached to. Instead of using “Direct” in the dialer field,
substitute a dialer entry for your modem. For example:
USR-SPORTSTER cuh00 19200
In this example, the Dialers file would have a “USR-SPORTSTER”
dialer entry to use to dial out. The modem would be attached to a serial
port which is accessed through device “cuh00” with a DTE speed of
19200.
How Automatic Failover Works
By default, pppd asks the peer to send LCP LinkQualityReport
messages. When the dedicated line fails, pppd stops receiving the
reports. pppd terminates the connection when the lack of Link Quality
Reports drives measured link quality below the configured threshold.
After unsuccessfully attempting to reestablish the connection on the
same line, pppd automatically fails over to the second entry in the
Systems file, and uses the modem to dial up and reestablish IP traffic.
Note that if the dedicated connection is restored, you must manually
cause the dialup modem to hang up the line. Then pppd attempts to