hi, heres a configuration that works to have fw with public ip and the cisco router doing only the ppoe authentication, vlan X is trunked to the firewall and the public ip is used as gateway for the firewall, vlan Y is used as management which is optional    Building configuration... Current configuration : 2224 bytes ! ! Last configuration change at 09:10:07 UTC Wed Nov 7 2018 by rob version 15.2 service config no service pad service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname mainint ! boot-start-marker boot-end-marker ! ! enable secret xxxxxxxxxxx enable password xxxxxxxxxxxxx ! no aaa new-model no process cpu extended history no process cpu autoprofile hog wan mode dsl ! ! ! ! ! ! ip domain name xxxxxxxxxx ip cef no ipv6 cef ! ! ! ! ! username xxxxxxx password xxxxxxxxxxx ! ! controller VDSL 0 operating mode vdsl2 ! ! ! ! ! interface ATM0 no ip address no atm ilmi-keepalive ! interface Ethernet0 no ip address no cdp enable ! interface Ethernet0.11 encapsulation dot1Q 11 pppoe-client dial-pool-number 1 ! interface FastEthernet0 switchport trunk native vlan Y switchport trunk allowed vlan X,Y switchport mode trunk no ip address no cdp enable spanning-tree portfast ! interface FastEthernet1 no ip address shutdown no cdp enable ! interface FastEthernet2 no ip address shutdown no cdp enable ! interface FastEthernet3 no ip address shutdown no cdp enable ! interface GigabitEthernet0 no ip address shutdown duplex auto speed 100 no cdp enable ! interface Vlan1 no ip address ! interface VlanY(management) ip address 192.168.0.8 255.255.255.0  ( example can be any local ip ) ! interface VlanX ip address x.x.x.x(pub ip) x.x.x.x(netmask) ! interface Dialer1 ip unnumbered VlanX encapsulation ppp dialer pool 1 ppp authentication pap chap ms-chap callin ppp chap hostname xxxxxxx ppp chap password  xxxxxx ppp pap sent-username xxxxxx password xxxxxx no cdp enable ! ip forward-protocol nd no ip http server no ip http secure-server ! ip route 0.0.0.0 0.0.0.0 Dialer1 ! no service-routing capabilities-manager ! ! control-plane ! ! line con 0 exec-timeout 0 0 no modem enable line aux 0 line vty 0 4 password xxxxxxxxxx login local transport input ssh ! scheduler allocate 60000 1000 ! end ... View more