Examkin

Question

Difficulty: Very hardOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router, R4, has the following interface states and IPv4 configurations:

- Loopback0: 192.168.250.1/24 (line protocol is down)
- Loopback1: 172.20.100.1/24 (line protocol is up)
- Loopback2: 10.100.1.1/24 (line protocol is up)
- GigabitEthernet0/0: 192.168.1.1/30 (line protocol is up)
- GigabitEthernet0/1: 10.254.254.1/30 (line protocol is up)

An engineer initially configures the router with the following commands:

text
router ospf 100
router-id 1.1.1.1

Later, the engineer enters subconfiguration mode and executes `no router-id 1.1.1.1`. To force the OSPF process to recalculate its router ID, the engineer executes `clear ip ospf process` and confirms the action.

What IPv4 address will R4 select as its operational OSPFv2 router ID?

Answer: 172.20.100.1

Answer

172.20.100.1
When the explicit `router-id` command is removed and the OSPF process is cleared using `clear ip ospf process`, OSPF fallback election logic applies. OSPF selects the highest IPv4 address among operational (up/up) loopback interfaces. Although Loopback0 has the highest loopback IP (192.168.250.1), its line protocol is down, excluding it from election. Between active loopbacks Loopback1 (172.20.100.1) and Loopback2 (10.100.1.1), Loopback1 has the higher numerical IPv4 address (172.20.100.1).

Step-by-Step Solution

1
Evaluate explicit router-id configuration
The explicit `router-id 1.1.1.1` command was negated via `no router-id 1.1.1.1`.
When the explicit router ID command is removed and the OSPF process is cleared, OSPF reverts to automatic selection logic.
2
Identify operational loopback interfaces
Loopback1 (172.20.100.1) and Loopback2 (10.100.1.1) are operational (up/up). Loopback0 (192.168.250.1) is down.
OSPF router ID selection prioritizes active (up/up) loopback interfaces over physical interfaces, but inactive/down interfaces cannot be selected regardless of IP value.
3
Compare active loopback IPv4 addresses to select highest IP
Comparing 172.20.100.1 and 10.100.1.1 yields 172.20.100.1 as the highest IPv4 address.
Among all operational loopback interfaces, the highest numerical IPv4 address is selected as the router ID upon process restart.

Key Concept

OSPFv2 Router ID Selection Priority Logic
Estimated Time:1m 30s
Rate this question

Topics

OSPFv2 Neighbor Adjacencies and Router ID

More questions from this topic