200-301 Question #1665: Real Exam Question with Answer & Explanation

Question

Lab Simulation 52 Please use the "Tasks" and "Topology" tabs to complete this lablet. Topology Tasks Refer to the topology. All physical cabling is in place. Routers 1 and 3 are accessible. Configure OSPF routing for the network and ensure R2 has joined Area 0 but does not participate in the DR/BDR process. Task 1 Configure OSPF on R2 and ensure R1 and R3 become neighbors. - use process ID 10 - use Lo0 IP as the router ID - advertise connected networks using an inverted subnet mask to match the exact prefixes used Task 2 Ensure R2 does not participate in the DR/BDR election Process. Answer: See the below explanation

Options

• taskConfigure OSPF routing for the network, ensuring R2 joins Area 0 and does not participate in the DR/BDR process.
• prerequisitesRouters 1 and 3 are accessible.

Explanation

OSPF Lab Simulation 52 — Explanation

Overall Goal

Configure OSPF (Open Shortest Path First) on R2 so it forms neighbor relationships with R1 and R3, exchanges routing information in Area 0 (the backbone area), but never wins the DR/BDR election on any of its broadcast segments. This is a common design choice when a router is a transit/middle node and you want more capable routers (R1 and R3) to handle the DR/BDR roles.

Step-by-Step Breakdown

Step 1: router ospf 10

What it does: Enters OSPF router configuration mode and starts OSPF process 10.

Why necessary: OSPF process IDs are locally significant — they identify which OSPF instance you're configuring on this router. The process ID must match what the lab requires (10), though it doesn't need to match on neighboring routers.

If skipped: OSPF never starts. No neighbors form.

Step 2: router-id 10.2.2.2

What it does: Manually sets the OSPF Router ID to the loopback address 10.2.2.2 (Lo0's IP).

Why necessary: The Router ID is a 32-bit identifier that uniquely identifies this router in the OSPF domain. Using the loopback is best practice because loopbacks are always up — a physical interface going down won't cause an OSPF Router ID change and force a re-convergence. The lab explicitly requires this.

If skipped: OSPF auto-selects a Router ID (highest loopback or highest active interface IP), which may not match what the lab expects, causing verification to fail.

Note: If OSPF is already running, you must do clear ip ospf process for a new router-id to take effect.

Step 3: network 10.0.2.0 0.0.0.31 area 0

What it does: Enables OSPF on any interface whose IP falls within 10.0.2.0–10.0.2.31, and advertises that prefix into Area 0.

Why necessary: This matches R2's interface toward R1 (e0/0). The wildcard mask 0.0.0.31 corresponds to a /27 subnet (255.255.255.224). Using the exact prefix wildcard ensures only the precise network is advertised — not a supernet.

If skipped: R2's link to R1 won't run OSPF; R1 neighbor relationship won't form.

Step 4: network 10.0.23.0 0.0.0.15 area 0

What it does: Enables OSPF on R2's interface toward R3 (e0/1) and advertises the 10.0.23.0/28 prefix into Area 0.

Why necessary: The wildcard 0.0.0.15 = /28 (255.255.255.240). This activates OSPF on the R2↔R3 link.

If skipped: R3 neighbor relationship won't form.

Step 5: network 10.2.2.2 0.0.0.0 area 0

What it does: Advertises the loopback interface (Lo0, 10.2.2.2/32) into OSPF Area 0.

Why necessary: A wildcard of 0.0.0.0 means "match this exact IP address" — it's a host mask for a /32. This makes the loopback reachable from R1 and R3. Loopbacks are also used as router IDs and management addresses, so advertising them is standard practice.

If skipped: The loopback network won't appear in the OSPF topology; other routers can't reach R2's Lo0.

Step 6–9: Interface Priority to 0

interface e0/0
 ip ospf priority 0
interface e0/1
 ip ospf priority 0

What it does: Sets OSPF priority to 0 on both of R2's physical interfaces.

Why necessary: On broadcast/multi-access networks, OSPF elects a Designated Router (DR) and Backup Designated Router (BDR) to reduce flooding overhead. Priority values determine who wins:

• Highest priority → DR
• Second highest → BDR
• Priority 0 → permanently ineligible — the router never participates in the election

This guarantees R2 will never become DR or BDR, leaving those roles to R1 and R3.

If skipped: R2 could win the DR/BDR election (especially if it has the highest Router ID), violating the lab requirement. R2 being DR also means if it goes down, the entire segment loses its flooding hub — undesirable for a transit router.

Why both interfaces? Each broadcast segment holds its own DR/BDR election. R2 has two such segments (toward R1 and toward R3), so both must be set to priority 0.

What Goes Wrong If Steps Are Out of Order

Memory Tip

"RINP-P" — Router OSPF, ID, Networks (3), Priority 0 per interface.

Or think of it as three phases:

1. Start OSPF (process + router ID)
2. Advertise networks (one network command per subnet, exact wildcard)
3. Disable DR eligibility (priority 0 on every broadcast interface)

The wildcard mask math: subtract the subnet mask from 255.255.255.255.

• /27 → 255.255.255.224 → wildcard = 0.0.0.31
• /28 → 255.255.255.240 → wildcard = 0.0.0.15
• /32 → 255.255.255.255 → wildcard = 0.0.0.0

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