Trouble-Shooting
　　Everything was fine before lunch:
　　Router_A
　　Router_A#sh run
　　Building configuration...
　　Current configuration:
　　!
　　version 11.2
　　no service password-encryption
　　no service udp-small-servers
　　no service tcp-small-servers
　　!
　　hostname Router_A
　　!
　　interface Ethernet0
　　ip address 172.16.4.17 255.255.255.240
　　!
　　interface Ethernet1
　　no ip address
　　shutdown
　　!
　　interface Serial0
　　ip address 10.16.0.5 255.255.255.252
　　no fair-queue
　　!
　　interface Serial1
　　ip address 10.16.1.5 255.255.255.252
　　ip rip authentication mode 0
　　!
　　router ospf 100
　　network 10.16.0.0 0.0.0.255 area 0
　　network 10.16.1.0 0.0.0.255 area 1
　　area 1 virtual-link 10.16.1.6 ...
　　message-digest-key 10 md5 Router_C
　　!
　　no ip classless
　　!
　　line con 0
　　exec-timeout 0 0
　　line aux 0
　　line vty 0 4
　　login
　　!
　　end
　　Router_A#sh ip ospf vir
　　Router_A#sh ip ospf virtual-links
　　Virtual Link OSPF_VL0 to router 10.16.1.6 is up
　　Run as demand circuit
　　DoNotAge LSA allowed.
　　Transit area 1, via interface Serial1, ...
　　Cost of using 64
　　Transmit Delay is 1 sec, State POINT_TO_POINT,
　　Timer intervals configured, Hello 10, Dead 40, ...
　　Wait 40, Retransmit 5
　　Hello due in 00:00:02
　　Adjacency State FULL (Hello suppressed)
　　Router_A#sh ip ospf vir
　　Router_A#sh ip ospf virtual-links
　　Virtual Link OSPF_VL0 to router 10.16.1.6 is down
　　Run as demand circuit
　　DoNotAge LSA allowed.
　　Transit area 1, Cost of using 65535
　　Transmit Delay is 1 sec, State DOWN,
　　Timer intervals configured, Hello 10, Dead 40, ...
　　Wait 40, Retransmit 5
　　Router_C