This question is stupid.
B, C and D all work. Tested on two 1941 routers running on IOS 15.1.
This is due to the fact the network command just specifies interfaces that run OSPF and due to the wildcards all three options have 20.1.1.2 inside their range.
B is a /32 and simpoly the IP
C will be automaticly converted to 0.0.0.0 255.255.255.255
D will be automaticly converted to 20.1.0.0 0.0.255.255
For OSPF routers to successfully establish a neighbor relationship, their directly connected interfaces must be in the same OSPF area. In this case, you only need to advertise the network 20.1.1.2/32 which corresponds to the interface FastEthernet 1/0, in the same area (Area 0) as Router R1.
The command to use is: network 20.1.1.2 0.0.0.0 area 0
As stated by others, B, C, and D all work, but the question is asking to just form a neighborship. B is The one that specifically targets that directive and no more.
Tested in GNS3 as seen below
R2(config)#router ospf 2
R2(config-router)#network 20.1.1.2 0.0.0.0 area 0
R2(config-router)#
*Mar 1 00:10:02.131: %OSPF-5-ADJCHG: Process 2, Nbr 1.1.1.1 on FastEthernet0/0 from LOADING to FULL, Loading Done
R2(config-router)#end
*Mar 1 00:10:07.555: %SYS-5-CONFIG_I: Configured from console by console
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:38 20.1.1.1 FastEthernet0/0
R2#
So B is correct!
No. The network command has nothing to do with the subnet matching. The IP addresses are both on the same subnet (20.1.0.0/24), so that requirement is met. Wildcard masks are a separate topic and have no effect on that rule. The wildcard masks on two routers don't have to match, as long as the result of the mask causes the correct interfaces to be included (on both sides).
No. The IP subnet (of the interface IP address) is what has to match. That has nothing to do with the wildcard mask in the "network" command. The routers don't see each other's wildcard mask, and they are not compared, and there is absolutely no rule saying they have to match.
I agree the B is correct. Because 20.1.1.2 is an individual IP vs. a /24 subnet like 21.1.1.0, then it would be appropriate to have an exact match wildcard mask (i.e. 0.0.0.0).
The network 20.0.0.0 0.0.0.255 area (” command on R2 did not cover the IP address of Fal/1 interface of R2 so OSPF did not run on this interface. Therefore we have to use the command “network 20.1.1.2 0.0.255.255 area 0” to turn on OSPF on this interface.
Note: The command “network 20.1.1.2 0.0.255.255 area 0” can be used too so this answer is also correct but answer B is the best answer here.
If you use 0.0.255.255 for the mask, that is /16, so you would just put 20.1.0.0 for the address. Putting 20.1.1.2 with a /16 mask doesn't make sense. But it's harmless, and most Cisco routers will automatically zero out the unneeded bits, and change it to 20.1.0.0 in the running config.
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