configure terminal
hostname R3
interface loopback 3
description Accounting Department
ip address 10.1.3.1 255.255.255.0
interface loopback 100
ip address 192.168.100.1 255.255.255.0
interface loopback 101
ip address 192.168.101.1 255.255.255.0
interface loopback 102
ip address 192.168.102.1 255.255.255.0
interface loopback 103
ip address 192.168.103.1 255.255.255.0
interface serial 0/0/1
ip address 10.1.23.3 255.255.255.0
no shutdown

router ospf 1
network 10.1.23.0 0.0.0.255 area 23
network 10.1.3.0 0.0.0.255 area 23
exit
interface loopback 3
ip ospf network point-to-point

router ospf 1
network 192.168.100.0 0.0.3.255 area 100
exit
interface loopback 100
ip ospf network point-to-point
interface loopback 101
ip ospf network point-to-point
interface loopback 102
ip ospf network point-to-point
interface loopback 103
ip ospf network point-to-point

router ospf 1
area 23 virtual-link 10.1.2.1

router ospf 1
area 100 range 192.168.100.0 255.255.252.0

interface serial0/0/0
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 cisco