Eu resolvi o problema. Estou respondendo a minha própria pergunta para referência futura.
Etapas:
1. Connect DSL connection using network manager
2. Connect eth0 interface: sudo udhcpc -i eth0
3. To check current routing: route -n
4. Two interface introduce two default gateway. To access internet remove local default gateway: sudo route del default gw 172.100.0.1 (for my network)
5. Add routing rule to access ftp servers(172.100.0.0/24 for my network): sudo route add -net 172.100.0.0/24 gw 172.100.0.1 eth0
Depois de seguir as etapas acima, eth0
e pppoe0
ambos devem ter IP
endereço.