Single Area
-----------

interface loopback 0                       ! OSPF uses Loopback as router ID
 ip address 198.6.30.254 255.255.255.255
!
interface ethernet 0
 ip address 192.5.1.1 255.255.255.0
 ip ospf message-digest key 1 md5 <passwd> ! authentication key for OSPF
!
interface ethernet 1
 ip address 198.6.30.1 255.255.255.240
!
router ospf 1
 log-adjacency-changes                     ! send adjacency changes to log
 passive-interface default                 ! all interfaces don't talk OSPF...
 no passive-interface ethernet 0           ! ...except for this one
 network 192.5.1.0 0.0.0.255 area 0        ! eth 0 net into OSPF
 network 198.6.30.0 0.0.0.15 area 0        ! eth 1 net into OSPF
 network 198.6.30.254 0.0.0.0 area 0       ! loop0 net into OSPF
 area 0 authentication message-digest      ! auth for area 0
!
no logging console                         ! switch off console messages
logging buffer 4096                        ! log messages in memory buffer
!

Two Areas
---------

interface loopback 0
 ip address 198.6.30.254 255.255.255.255
!
interface ethernet 0
 ip address 192.5.1.1 255.255.255.0
 ip ospf message-digest key 1 md5 <passwd> ! authentication key for OSPF
!
interface ethernet 1
 ip address 198.6.30.1 255.255.255.240
 ip ospf message-digest key 1 md5 <passwd> ! authentication key for OSPF
!
router ospf 1
 log-adjacency-changes
 passive-interface default                 ! all interfaces don't talk OSPF...
 no passive-interface ethernet 0           ! ...except for this one...
 no passive-interface ethernet 1           ! ...and this one
 network 192.5.1.0 0.0.0.255 area 0
 network 198.6.30.0 0.0.0.15 area 10       ! eth1 is in area 10
 network 198.6.30.254 0.0.0.0 area 0
 area 0 authentication message-digest
 area 10 authentication message-digest     ! authentication for area 10
!
no logging console
!

Three Areas
-----------

interface loopback 0
 ip address 198.6.30.254 255.255.255.255
!
interface ethernet 0
 ip address 192.5.1.1 255.255.255.0
 ip ospf message-digest key 1 md5 <passwd>
!
interface ethernet 1
 ip address 198.6.30.1 255.255.255.240
 ip ospf message-digest key 1 md5 <passwd>
!
interface serial 0
 ip address 80.252.10.2 255.255.255.252
 ip ospf message-digest key 1 md5 <passwd>
!
router ospf 1
 log-adjacency-changes
 passive-interface default
 no passive-interface ethernet 0
 no passive-interface ethernet 1
 no passive-interface serial 0
 network 192.5.1.0 0.0.0.255 area 0
 network 198.6.30.0 0.0.0.15 area 10
 network 198.6.30.254 0.0.0.0 area 0
 network 80.252.10.0 0.0.0.3 area 42
 area 0 authentication message-digest
 area 10 authentication message-digest
 area 42 authentication message-digest
!
no logging console
!

Single Area + Default Originate
-------------------------------

interface loopback 0
 ip address 198.6.30.254 255.255.255.255
!
interface ethernet 0
 ip address 192.5.1.1 255.255.255.0
 ip ospf message-digest key 1 md5 <passwd>
!
interface ethernet 1
 ip address 198.6.30.1 255.255.255.240
!
interface serial 0
 description External connection to another ISP
 ip address 80.240.10.1 255.255.255.252
!
router ospf 1
 log-adjacency-changes
 passive-interface default                ! no interfaces talk OSPF...
 no passive-interface ethernet 0          ! ...apart from this one
 default-information originate always     ! always send default route
 network 192.5.1.0 0.0.0.255 area 0
 network 198.6.30.0 0.0.0.15 area 0
 network 198.6.30.254 0.0.0.0 area 0
 network 80.240.10.0 0.0.0.3 area 0       ! serial 0 net in OSPF
 area 0 authentication message-digest
!
ip route 0.0.0.0 0.0.0.0 Serial 0         ! static default to upstream
!
no logging console
!