Cisco CCNA mock exam questions sample test – Question 61

.Which of the following commands would instruct OSPF to advertise ONLY the 192.168.10.0/24 network in Area 0?

A. Router(config)# router ospf 1
Router(config-router)# network 192.168.10.0 0.0.0.255 area 0
B. Router(config)# router ospf 1
Router(config-router)# network 192.168.11.0 0.0.0.255 area 0
C. Router(config)# router ospf 1
Router(config-router)# network 192.168.10.0 255.255.255.0 area 0
D. Router(config)# router ospf 1
Router(config-router)# network 192.168.10.0 0.0.255.255 area 0

Correct Answer: A

Explanation:
The command Router(config-router)# network 192.168.10.0 0.0.0.255 area 0 would instruct OSPF to advertise the 192.168.10.0 network in Area 0. It is executed in OSPF process 1
configuration mode, as indicated by the prompt Router(config-router)#. This command correctly states the network as 192.168.10.0 and uses the proper wildcard mask of 0.0.0.255.
The command Router(config-router)# network 192.168.11.0 0.0.0.255 area 0 is incorrect because it advertises the 192.168.11.0/24 network instead of the 192.168.10.0/24 network.
The command Router(config-router)# network 192.168.10.0 255.255.255.0 area 0 is incorrect because it uses a regular mask instead of a wildcard mask.
The wildcard mask in OSPF network statements must be expressed inversely, and not as a regular subnet mask. If the network you are configuring for OSPF operation is
192.168.10.0/24, then the inverse version of a /24 mask (or 255.255.255.0) would be 0.0.0.255. The correct command, Router(config-router)# network 192.168.10.0 0.0.0.255 area
0,will configure OSPF to run over any local interfaces assigned an IP address beginning with 192.168.10, since the inverse mask dictates that the first three octets must be a match.
The command Router(config-router)# network 192.168.10.0 0.0.255.255 area 0 is incorrect because it uses an improper wildcard mask. This mask would instruct OSPF to advertise
any network with a prefix longer than the 192.168.0.0/16 network.
When routing does not seem to be working correctly, one of the first things to check is whether OSPF is operating on the proper interfaces. OSPF is enabled by network statements.
To verify the network statements that were entered, you should execute the show run command and examine the output. If the network statement is configured so that the interface on
the router is not in that network, OSPF will not operate on that interface. For example, suppose that Router A has an interface of 192.168.5.1/30 and the show run command produces
the following output:

router ospf 2 area 0
network 192.168.5.0 0.0.0.4
In this case, OSPF will not operate on the interface because the router interface is not in the network indicated by the network statement. The problem is not the network address but
the wildcard mask. For a 30-bit mask, the wildcard should be 0.0.0.3, not 0.0.0.4. The wildcard mask can be determined by subtracting the regular mask value in the last octet (252)
from 255, which is 3. The solution would to remove the incorrect statement and enter the correct statement as follows:
routerA(config)# router ospf 2 area 0
no network 192.168.5.0 0.0.0.4 area 0
network 192.168.5.0 0.0.0.3 area 0
Objective:
Routing Fundamentals
Sub-Objective:
Configure, verify, and troubleshoot single area and multi-area OSPFv2 for IPv4 (excluding authentication, filtering, manual summarization, redistribution, stub, virtual-link, and LSAs)
References:
Cisco > Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4T > Part 6: OSPF > Configuring OSPF > OSPF Configuration Task List > Enabling OSPF