Although most implementations of PPP occur over aysnchronous
dial up connections, PPP can be used for synchronous transmission
over high speed serial interfaces. It can also be used on dedicated
lines and constantly open telephone lines. The latter is a dial-up
connection, but it is not on-demand.
Synchronous PPP |
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PPP can run in synchronous mode using
a high speed serial interface at line speeds up to T1 (1.544Mb/s).
To prepare your system to use a high speed interface, follow the
instructions in the hardware installation guide.
Dedicated Lines |
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Use pppd's
dedicated argument if the PPP implementation uses asynchronous serial
connections that are always available. These connections often
use high-speed asynchronous short-haul modems over a building or
campus wiring plant. The dedicated argument instructs pppd
to never give up on the connection. If the peer tells pppd
to disconnect, pppd will continuously
attempt to reconnect and connectivity is reestablished as soon as
possible if one end of the link goes down.
If there is a fatal disconnect, through LQM failures or loss
of the Carrier Detect signal, pppd
closes the device and consults the Systems
file to find another matching entry. If none is available, pppd
waits for the call retry delay to pass and tries the original connection
again. Normally, no getty or login process is run on a dedicated
line device and both ends of the circuit actively try to connect
to their peer. Each machine's Autostart
script should contain a line like the following:
pppd local:remote auto dedicated |
The Systems file should specify
a device name like cuh00 in the device field. ACU, for "Any
Call Unit", should not be used. For example:
The Devices file should contain
a line like the following:
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 |
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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 reconnect using the next entry in Systems,
or, if no additional entries exist, pppd
wraps around to the first entry in the file which is the dedicated
connection.
Constantly-Open Telephone Calls |
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Some PPP connections are always
up. The system does not use pppd's
on-demand dialing to reestablish a link for new traffic. This is
not the same as using a dedicated line, because modems on constantly
open connections must be dialed, or a login negotiated, before PPP
frames can be exchanged. In the "constantly up"
situation, use the "up" argument on pppd's
command line. The argument "up" instructs pppd
to make every effort to keep the connection up. For example, when
the connection goes down, pppd
immediately redials the modem, rather than waiting for traffic demand.
Do not use the up and idle arguments together.
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