Manual reroute traffic 'ip ospf cost' better than auto reroute

Router1#sh ip os int b
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Vl50         1     0               161.1.79.7/24      300   DR    1/1
Fa0/15       1     0               161.1.78.7/24      100   BDR   1/1
Lo0          1     0               150.1.7.7/24       1     LOOP  0/0

Router2#sh ip os int b
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Fa0/16       1     0               161.1.89.8/24      150   DR    1/1
Fa0/15       1     0               161.1.78.8/24      100   DR    1/1
Lo0          1     0               150.1.8.8/24       1     LOOP  0/0

Router3#sh ip os int b
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Fa0/16       1     0               161.1.89.9/24      150   BDR   1/1
Vl50         1     0               161.1.79.9/24      300   BDR   1/1
Lo0          1     0               150.1.9.9/24       1     LOOP  0/0

Router3#sh ip ro 150.1.7.7
Routing entry for 150.1.7.7/32
  Known via "ospf 1", distance 110, metric 251, type intra area
  Last update from 161.1.89.8 on FastEthernet0/16, 00:04:39 ago
  Routing Descriptor Blocks:
  * 161.1.89.8, from 150.1.7.7, 00:04:39 ago, via FastEthernet0/16
      Route metric is 251, traffic share count is 1

Router3#traceroute 150.1.7.7
Type escape sequence to abort.
Tracing the route to 150.1.7.7
  1 161.1.89.8 0 msec 4 msec 0 msec
  2 161.1.78.7 4 msec *  0 msec

# SCENARIO I, Change cost with 'ip ospf cost 50'

Router3(config)#do sh ip os int b
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Fa0/16       1     0               161.1.89.9/24      150   BDR   1/1
Vl50         1     0               161.1.79.9/24      50    BDR   1/1
Lo0          1     0               150.1.9.9/24       1     LOOP  0/0

Router3(config)#do sh ip ro 150.1.7.7
Routing entry for 150.1.7.7/32
  Known via "ospf 1", distance 110, metric 51, type intra area
  Last update from 161.1.79.7 on Vlan50, 00:00:36 ago
  Routing Descriptor Blocks:
  * 161.1.79.7, from 150.1.7.7, 00:00:36 ago, via Vlan50
      Route metric is 51, traffic share count is 1

Router3(config)#do traceroute 150.1.7.7
Type escape sequence to abort.
Tracing the route to 150.1.7.7
  1 161.1.79.7 4 msec *  0 msec

>> Verify with ping, during I change cost 50

Router3#ping 150.1.7.7 repeat 10000
Success rate is 100 percent (10000/10000), round-trip min/avg/max = 1/1/28 ms

# SCENARIO II, Force shutdown interface FastEternet0/15 in Router1 (Router1-Router2)
traffic from Router3 to Router 1 should be route to interface vlan 50 (Router1-Router3)

Router3(config-if)#do sh ip ro 150.1.7.7
Routing entry for 150.1.7.7/32
  Known via "ospf 1", distance 110, metric 301, type intra area
  Last update from 161.1.79.7 on Vlan50, 00:00:29 ago
  Routing Descriptor Blocks:
  * 161.1.79.7, from 150.1.7.7, 00:00:29 ago, via Vlan50
      Route metric is 301, traffic share count is 1

Router3(config-if)#do traceroute 150.1.7.7
Type escape sequence to abort.
Tracing the route to 150.1.7.7
  1 161.1.79.7 0 msec *  0 msec

>> Verify with ping, during I force shutdown

Router3#ping 150.1.7.7 repeat 10000
Success rate is 99 percent (9995/10000), round-trip min/avg/max = 1/1/216 ms