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Null0 route in OSPF

                Hi guys, I would like to know why the null0 route is eliminated by using "no discard-route external"  if this route has been generated internally by my ospf process. I just was trying to respond a question from the CCIE exam and I came across with this thing. I have heard that you can remove the null route by issuing no "ip discard-route internal" command if the route was internally generated. Please have a look to those configurations and outputs and provide me your best answer. The topology that I have is like this, and it is supposed that I have to create a loop within R1 an R2 by summaring the 192.168.0.0/22 in the R1 side and 192.168.0.0/21 in R2 under each ospf process respectively:

Topology.PNG

R1

router ospf 200

log-adjacency-changes

summary-address 192.168.0.0 255.255.248.0

redistribute connected subnets

network 10.1.2.0 0.0.0.31 area 0

R1#
*Mar  1 02:55:42.907: %SYS-5-CONFIG_I: Configured from console by console
R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/27 is subnetted, 1 subnets
C       10.1.2.0 is directly connected, Serial0/0
C    192.168.1.0/24 is directly connected, Loopback0
O    192.168.0.0/22 is a summary, 00:00:04, Null0
O E2 192.168.0.0/21 [110/20] via 10.1.2.2, 00:00:07, Serial0/0

------------------------------------------------------------------------------------------------------------

R1 Again after the "no discard-route internal command:

router ospf 200

log-adjacency-changes

no discard-route internal

summary-address 192.168.0.0 255.255.252.0

redistribute connected subnets

network 10.1.2.0 0.0.0.31 area 0

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/27 is subnetted, 1 subnets
C       10.1.2.0 is directly connected, Serial0/0
C    192.168.1.0/24 is directly connected, Loopback0
O    192.168.0.0/22 is a summary, 00:02:58, Null0  <----------- It's still there
O E2 192.168.0.0/21 [110/20] via 10.1.2.2, 00:01:01, Serial0/0

--------------------------------------------------------------------------------------------------------------

R1 after the "no discard-route external" command:

router ospf 200

log-adjacency-changes

no discard-route external

summary-address 192.168.0.0 255.255.252.0

redistribute connected subnets

network 10.1.2.0 0.0.0.31 area 0

R1#sh ip route (NO NULL ROUTE)
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/27 is subnetted, 1 subnets
C       10.1.2.0 is directly connected, Serial0/0
C    192.168.1.0/24 is directly connected, Loopback0
O E2 192.168.0.0/21 [110/20] via 10.1.2.2, 00:02:59, Serial0/0

Thank you in advance for any replies!!