A network administrator configures OSPF on a Cisco router with the following commands: Router (config) #router ospf 1 and Router (config-router) #network 10.0.0.0 255.0.0.0 area 0. However, no OSPF routes appear in the IP routing table. Which configuration error most likely explains why the router is not forming OSPF adjacencies or installing OSPF routes?

Introduction / Context:
Open Shortest Path First (OSPF) is a link state routing protocol that uses the network command and wildcard masks to determine which interfaces participate in OSPF. If the network command is incorrect, OSPF will not enable on any interfaces, and the router will not form adjacencies or install OSPF routes. The question describes a common configuration scenario where OSPF appears to be configured, but no OSPF routes show up in the routing table. Understanding how wildcard masks work in OSPF network statements is essential for troubleshooting this type of problem.

Given Data / Assumptions:

• OSPF has been enabled with router ospf 1, so an OSPF process exists.
• The network statement used is network 10.0.0.0 255.0.0.0 area 0.
• No OSPF routes appear in the routing table.
• The 10.0.0.0 network actually exists on one or more router interfaces.
• There is at least one neighboring OSPF router available to form adjacencies, assuming correct configuration.

Concept / Approach:
In OSPF, the second parameter in the network command is a wildcard mask, not a subnet mask. A wildcard mask is the inverse of a subnet mask. For example, a Class A network 10.0.0.0 with a subnet mask of 255.0.0.0 would use a wildcard mask of 0.255.255.255. If you mistakenly configure network 10.0.0.0 255.0.0.0 area 0, OSPF interprets this as a wildcard mask of 255.0.0.0, which does not match the actual interface addresses. As a result, OSPF is never enabled on any interfaces, and no adjacencies or routes appear. The process ID and area number are locally significant and typically do not prevent OSPF from running if they are syntactically valid.

Step-by-Step Solution:
1. Recognize that the network command syntax is network network-address wildcard-mask area area-id. 2. Note that the configuration uses 255.0.0.0 where a wildcard mask is expected. 3. Convert the intended subnet mask 255.0.0.0 to its correct wildcard mask, which is 0.255.255.255. 4. See that network 10.0.0.0 255.0.0.0 area 0 is therefore wrong, and should be network 10.0.0.0 0.255.255.255 area 0. 5. Understand that with the incorrect wildcard mask, no interfaces are matched, so OSPF does not run on any interface. 6. Conclude that the main configuration error is the incorrect wildcard mask in the network statement.

Verification / Alternative check:
You can verify this reasoning by remembering that OSPF process IDs are locally significant and can be any number from 1 to 65535. The area 0 is also valid, as it is the backbone area and is commonly used. No explicit autonomous system number is configured directly under OSPF on Cisco IOS in the same way as EIGRP, so an AS misconfiguration does not apply here. The only parameter that clearly stands out as wrong is the wildcard mask. When you mentally replace 255.0.0.0 with 0.255.255.255, the configuration becomes correct and would allow interfaces in the 10.0.0.0 network to join area 0 and form adjacencies.

Why Other Options Are Wrong:
Option b: The OSPF area 0 is valid and required for the backbone, so this is not an error. Option c: The OSPF process ID of 1 is acceptable. Process IDs are locally significant and do not need to match neighbors. Option d: OSPF does not use an autonomous system number configuration in the same simple way as EIGRP, so AS configuration is not the issue in this context. Option e: OSPF can form adjacencies and install routes even if a manual router-id is not set, because it can choose one based on highest loopback or interface IP, so this is not the primary cause here.

Common Pitfalls:
A very common mistake for learners is to confuse subnet masks and wildcard masks. In routing protocols like OSPF, the network command accepts wildcard masks, whereas in interface configurations, you use subnet masks. If you accidentally paste a subnet mask into an OSPF network command, no interfaces may match and OSPF will appear to be nonfunctional even though the process is configured. To avoid this, always remember that wildcard mask equals bitwise inverse of the subnet mask and double check that zeros go where you want exact matching and 255 values represent bits you do not care about. This habit prevents many configuration errors in real networks and in exam questions like this one.

Final Answer:
The problem is that the administrator used a subnet mask instead of a wildcard mask, so the wildcard mask in the network command is incorrect.