https://help.ubuntu.com/community/ADSLPPPoE#Boot_issues

Exposing PPP Connections to the OS
Now, you could get by this far by using scripts to bring up your interfaces, 
or bring them up and down manually with the pon and poff commands. 
But there is a better way to handle the connection. 
Ubuntu already provides the ifup and ifdown commands for configuring your regular network connections. 
Well, the ifup and ifdown commands can be used to establish the PPP connections also. 
This makes the interfaces consistent with any other interface, 
and you are probably familiar with these commands already. 
This is also critical if the connection needs to be established at system boot.

Interfaces maintained the the ifup and ifdown commands are defined in the /etc/network/interfaces file. 
If you look at the file now, you may already see how your network connections are currently configured, 
be it by DHCP or static IP Address assignment
------------------------------
cat /etc/network/interfaces

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# DHCP network configuration
auto eth0
iface eth0 inet dhcp

******************************

We will modify this file to include a section for the PPP connection and to change eth0 from being configured by DHCP. 
The pppoeconf script created following configuration file:

------------------------------

cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

auto dsl-provider
iface dsl-provider inet ppp
        pre-up /sbin/ip link set dev eth1 up # line maintained by pppoeconf
        provider dsl-provider

auto eth0
iface eth0 inet manual

******************************

This allows the user to use ifup and ifdown to enable and disable the pppoeconf configured connection:

------------------------------

ifup dsl-provider
ifdown dsl-provider


******************************







Boot issues
The configuration provided by pppoeconf does not actually bring the connection up at system boot. 
But with some modifications to the configuration file, your system should dialup the connection.

Below is the template that will bring the connect up at system boot:

------------------------------
auto eth0
iface eth0 inet ppp
        pre-up /sbin/ip link set dev eth0 up
        provider dsl-provider
        post-down /sbin/ip link set dev eth0 down
******************************

Notice that the two sections have been merged together. 
There is no separate stanza for dsl-provider and eth0. Only one stanza to cover them both.

Here is the stanza for the SBCGlobal peer file from earlier in the walk-through:

------------------------------
auto eth2
iface eth2 inet ppp
        pre-up /sbin/ip link set dev eth2 up
        provider SBCGlobal
        post-down /sbin/ip link set dev eth2 down
******************************
Here the stanza configures eth2 (versus eth0), and it calls the SBCGlobal peer file (versus the default dsl-provider peer file).

With this stanza, the system will be able to establish the connection using both the ifup and ifdown commands, 
and at system startup/shutdown. This works for 9.10 (Karmic Koala). If it does not work for you, you may try the following.

If you find that you have to run pppoeconf each time you boot, you can try two things:

Edit /etc/network/interfaces as described here, so that that 'pppoe maintained' lines are before 'auto dsl-provider':

------------------------------
# added by pppoeconf
auto eth0
iface eth0 inet manual
pre-up /sbin/ip link set dev eth0 up # line maintained by pppoeconf

auto dsl-provider
iface dsl-provider inet ppp
provider dsl-provider
******************************

Failing that, edit /etc/rc.local, and before the last line ("exit 0"), add:

------------------------------
ip link set dev eth0 up
pon dsl-provider
******************************

Error logs
If you are having problems with your connection, you may find valuable information in the system message logs. 
You may access system logs either in a terminal, or with a graphical interface.

To use the graphical log viewer, in the menu bar, 
go to : System > Administration > System Log. You will find the system messages in /var/log/messages.

To use the terminal, type: 

sudo dmesg