200-301 Question #1621: Real Exam Question with Answer & Explanation

Question

Lab Simulation 44 Guidelines This is a lab item in which tasks will be performed on virtual devices. - Refer to the Tasks tab to view the tasks for this lab item. - Refer to the Topology tab to access the device console(s) and perform the tasks. - Console access is available for all required devices by clicking the device icon or using the tab(s) above the console window. - All necessary preconfigurations have been applied. - Do not change the enable password or hostname for any device. - Save your configurations to NVRAM before moving to the next item. - Click Next at the bottom of the screen to submit this lab and move to the next question. - When Next is clicked, the lab doses and cannot be reopened. Topology Tasks Refer to the topology. All physical cabling is in place. Routers 2 and 3 are inaccessible. Configure OSPF routing for the network and ensure R1 has joined Area 0 without using network statements. Task 1 Configure OSPF on R1 with a process ID and router-ID only as follows: - use process ID 30 - use E0/0 IP as the router ID Task 2 - Configure R1 to establish neighbor adjacencies with R2 and R3. The network statement under the OSPF process must not be used. - Configure R1 to always become the DR for Area 0 Answer: See the below explanation

Options

• taskConfigure OSPF on R1 with process ID 30 and E0/0 IP as router ID, establish neighbor adjacencies with R2 and R3 without using network statements, and configure R1 to always become the Designated Router (DR) for Area 0.
• prerequisites

Explanation

OSPF Lab Simulation 44 — Explained

Overall Goal

Configure OSPF on R1 so it participates in Area 0 without using network statements, and ensure R1 always wins the Designated Router (DR) election. The key constraint is using interface-level OSPF (ip ospf <process> area <area>) instead of the traditional network statement under the OSPF process.

Step-by-Step Breakdown

Step 1: router ospf 30

Starts an OSPF process with process ID 30. The process ID is locally significant — it doesn't need to match neighboring routers. Without this, none of the subsequent OSPF commands have a process to attach to.

Skipping this means no OSPF process exists and the interface commands in later steps will reference a non-existent process.

Step 2: router-id 10.0.12.1

Manually sets the OSPF Router ID to R1's E0/0 IP address. The task explicitly requires using E0/0's IP as the router ID.

OSPF uses the Router ID to:

• Uniquely identify the router in the OSPF domain
• Break ties in DR elections (higher RID wins when priority is equal)

Without this, OSPF would auto-select a Router ID from the highest loopback or active interface IP — unpredictable and not what the task requires.

Order matters: Set the router ID before OSPF adjacencies form. If set after, you must issue clear ip ospf process to apply it, disrupting adjacencies.

Step 3–4: interface E0/0 → ip ospf 30 area 0

This is the core of the "no network statement" requirement. Instead of:

network 10.0.12.0 0.0.0.255 area 0

You enable OSPF directly on the interface:

ip ospf 30 area 0

This tells OSPF process 30 to include this interface in Area 0 and begin sending Hello packets out E0/0 to discover R2.

Skipping this means R1 never sends Hellos out E0/0 and never forms an adjacency with R2.

Step 5: ip ospf priority 255 (on E0/0)

Sets the OSPF interface priority to 255 (maximum possible). During the DR election, the router with the highest priority wins. Default is 1; setting 255 guarantees R1 always becomes DR on the E0/0 segment.

If priority is left at default (1) and another router has priority 1 too, the DR is decided by Router ID — not guaranteed. Setting 255 makes R1 the definitive winner regardless.

Steps 6–7: interface E0/1 → ip ospf 30 area 0

Same as steps 3–4 but for E0/1, enabling OSPF on the link connecting to R3. Without this, R1 never forms adjacency with R3.

Step 8: ip ospf priority 255 (on E0/1)

Same rationale as step 5 — ensures R1 wins the DR election on the E0/1 segment as well.

What Happens If Steps Are Skipped

Memory Tips

• "Interface OSPF = no network statement" — ip ospf <pid> area <area> on the interface replaces the network command entirely.
• Priority 255 = always DR — Think of it as "255 = max = boss of the segment."
• Router ID first — Always set before adjacencies form, or you'll need to reset the process.
• Per-interface, not per-process — Priority and area are set on the interface, not under router ospf.

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