In a single OSPF area, if all routers have the same priority and no loopback interface is configured, which IP address will a router use as its OSPF Router ID?

Introduction / Context:
OSPF requires a unique Router ID on each router within an autonomous system. The Router ID is a 32 bit value that looks like an IP address. If you do not manually configure the Router ID, OSPF selects one automatically based on a set of rules. Knowing these rules is important for predicting OSPF behaviour and understanding why certain routers become designated routers in multiaccess networks.

Given Data / Assumptions:

• All routers in the OSPF area have the same priority value for elections.
• No loopback interfaces are configured on the router in question.
• The router has one or more active physical interfaces with IP addresses assigned.
• OSPF is allowed to choose the Router ID automatically according to its default rules.

Concept / Approach:
OSPF chooses the Router ID using a specific order of preference. First, if a Router ID is configured manually with the router-id command, that value is used. If no manual Router ID is configured, OSPF looks for loopback interfaces and chooses the highest IP address on any loopback interface. If no loopbacks exist, OSPF then selects the highest IP address on any active physical interface. This automatic selection ensures that each router has a stable and unique Router ID as long as interface addresses do not change.

Step-by-Step Solution:
Step 1: Confirm that there is no manually configured OSPF Router ID on the device.Step 2: Verify that there are no loopback interfaces configured; the question states this explicitly.Step 3: Identify all physical interfaces that are up and have IP addresses.Step 4: Among those interfaces, determine which one has the highest IP address numerically.Step 5: OSPF uses this highest physical interface IP address as the Router ID for this router.

Verification / Alternative check:
You can confirm the selected Router ID using the show ip ospf command on a Cisco router. The output clearly displays the Router ID. By comparing this value with the IP addresses on the router interfaces, you can verify that it matches the highest physical interface IP when no loopback or manual ID is present.

Why Other Options Are Wrong:
Option A is incorrect because OSPF prefers the highest, not the lowest, IP address when choosing from physical interfaces.Option C and Option D refer to logical interfaces such as subinterfaces or loopbacks, but the question explicitly states that no loopback exists and the rule for physical interfaces is more specific.Option E is wrong because the Router ID is not random; it follows deterministic selection rules based on configured IP addresses.

Common Pitfalls:
Some engineers assume that OSPF always uses the lowest IP address or that it always prefers loopbacks even when they are down. Remember that OSPF selects the highest IP on loopback interfaces if they exist, otherwise the highest IP on physical interfaces. Manually configuring the Router ID is often recommended in production to avoid changes when interface addressing is modified.

Final Answer:
In this scenario, the router uses The highest IP address of any physical interface as its OSPF Router ID.