Here is an example ISDN config for a Cisco 801 router
version 12.0 ! no ip domain-lookup isdn switch-type basic-net3 ! ! ! interface Ethernet0 ip address 172.16.1.1 255.255.255.0 no ip directed-broadcast ip nat inside ! interface BRI0 ip address negotiated no ip directed-broadcast ip nat outside encapsulation ppp dialer string (put ISDN phone number here) dialer-group 1 isdn switch-type basic-net3 no cdp enable ppp chap hostname (Put username here) ppp chap password (Put password here) ! router rip network 172.16.0.0 ! ip nat translation timeout 180 ip nat inside source list 1 interface BRI0 overload ip http server ip classless ip route 0.0.0.0 0.0.0.0 BRI0 ! access-list 1 permit 172.16.0.0 0.0.255.255 access-list 100 deny ip any host 172.16.255.255 access-list 100 permit ip any any dialer-list 1 protocol ip list 100 alias exec u undebug all !
