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:

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!!