Cisco CCNA Single-Area OSPFv2 Configuration Questions and Answers

Question 1

A network engineer configures a Cisco router for single-area OSPFv2. The router has three active interfaces: Loopback0 (10.0.0.1/32), GigabitEthernet0/0 (192.168.1.1/24), and GigabitEthernet0/1 (172.16.1.1/24). The OSPF process is started, but the `router-id` command is NOT used. Which IP address will OSPF select as the router ID?

Accepted Answer

10.0.0.1

Answer

The OSPF router ID selection process on Cisco IOS follows a specific order of precedence: 1) The address manually configured with the `router-id` command. 2) The highest IP address on any configured loopback interfaces. 3) The highest IP address on any active non-loopback (physical) interfaces. Since the `router-id` command was not used, OSPF will select the highest loopback IP address, which is 10.0.0.1.

Question 2

An administrator needs to enable OSPF on an interface with the IP address 192.168.10.65/27. Which of the following `network` commands will correctly and most precisely enable OSPF only on this specific interface?

Accepted Answer

network 192.168.10.65 0.0.0.0 area 0

Answer

The OSPF `network` command uses a wildcard mask, which is the inverse of a subnet mask. A wildcard mask of 0.0.0.0 tells the router to match all 32 bits of the specified IP address exactly. Therefore, `network 192.168.10.65 0.0.0.0 area 0` will enable OSPF exclusively on the interface configured with the IP address 192.168.10.65.

Question 3

A network administrator issues the `passive-interface default` command under the OSPF process. What is the effect of this command?

Accepted Answer

It stops OSPF from forming adjacencies on any interface.

Answer

The `passive-interface default` command globally prevents the router from sending or receiving OSPF Hello packets on all interfaces by default. This stops the formation of neighbor adjacencies. To form an adjacency on a specific interface, the administrator must then use the `no passive-interface <interface-name>` command. The networks on passive interfaces are still advertised into OSPF.

Question 4

Two OSPF routers are connected via a FastEthernet link (100 Mbps) but are failing to form a full adjacency, remaining in the 2-WAY state. Both routers are on the same subnet (10.1.1.0/30) and in the same area (Area 0). Which of the following is a valid reason for this issue on a broadcast network type like Ethernet?

Accepted Answer

The OSPF priority on both routers is set to 0.

Answer

On broadcast multi-access networks, OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR). Routers that are not a DR or BDR (DROTHERs) will only form a FULL adjacency with the DR and BDR. DROTHER routers will remain in the 2-WAY state with other DROTHERs. If both routers have their OSPF interface priority set to 0, neither can be elected as DR or BDR, preventing them from moving past the 2-WAY state.

Question 5

A company's network includes links with speeds of 1 Gbps and 10 Gbps. A network engineer notices that OSPF is treating both links as having the same cost. What command should be used within the OSPF routing process to ensure OSPF calculates a more granular and accurate cost for these high-speed links?

Accepted Answer

auto-cost reference-bandwidth 10000

Answer

By default, OSPF uses a reference bandwidth of 100 Mbps. The cost is calculated as (Reference Bandwidth / Interface Bandwidth). For any interface with a bandwidth of 100 Mbps or higher (like 1 Gbps and 10 Gbps), the calculated cost will be 1, making them appear equal. The `auto-cost reference-bandwidth <value_in_Mbps>` command increases the reference bandwidth, allowing for different costs to be calculated for links faster than 100 Mbps. For example, a value of 10000 (for 10 Gbps) would result in a cost of 10 for a 1 Gbps link and 1 for a 10 Gbps link.

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Cisco CCNA Single-Area OSPFv2 Configuration Questions and Answers