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Difficulty: Very hardOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router, R1, initializes its OSPFv2 routing process without an explicit router-id configuration. R1 has three interfaces with the following states:
- Loopback0: IP address 172.16.100.1/32 (Line protocol UP, Operational state UP)
- Loopback1: IP address 192.168.50.1/32 (Line protocol DOWN, Operational state DOWN)
- GigabitEthernet0/0: IP address 10.0.12.1/24 (Line protocol UP, Operational state UP, MTU 1500)

After the OSPF process starts operating and selects its Router ID, a network engineer enters the command 'router-id 1.1.1.1' under the OSPF configuration mode on R1, but does not reload the router or issue the 'clear ip ospf process' command.

R1 then sends an initial OSPF Hello packet out GigabitEthernet0/0 to neighboring router R2. Router R2 has its connected interface configured in the same OSPF area with matching timers, but with an interface MTU of 1400 bytes. When R2 receives this initial Hello packet from R1 (which does not yet list R2's Router ID in its neighbor field), which OSPF Router ID will R1 use in the packet header, and what state will R2 record for neighbor R1 upon processing this packet?

  1. Router ID: 172.16.100.1; Neighbor State on R2: InitAnswer
  2. B
    Router ID: 1.1.1.1; Neighbor State on R2: Init
  3. C
    Router ID: 192.168.50.1; Neighbor State on R2: 2-Way
  4. D
    Router ID: 172.16.100.1; Neighbor State on R2: ExStart

Answer

Router ID: 172.16.100.1; Neighbor State on R2: Init
The correct answer specifies Router ID 172.16.100.1 and Init state. R1 selects 172.16.100.1 because Loopback0 is the highest operational loopback interface when OSPF starts. Configuring a manual router-id later has no effect until the OSPF process is restarted. When R2 receives R1's first Hello packet (which lacks R2's Router ID in its neighbor list), R2 records R1 in the Init state. MTU differences do not prevent entering Init state.

Step-by-Step Solution

1
Determine the active OSPF Router ID on R1 at process initialization
Loopback0 (172.16.100.1) is chosen as the Router ID
OSPF Router ID selection logic evaluates: 1) manual 'router-id' setting, 2) highest IP address among operational (UP/UP) loopback interfaces, 3) highest IP address among operational non-loopback interfaces. Loopback1 is DOWN/DOWN, so it is ignored. Loopback0 (172.16.100.1) is the highest active loopback IP.
2
Evaluate the impact of configuring 'router-id 1.1.1.1' after process startup
The active Router ID remains 172.16.100.1
OSPF Router ID election is non-preemptive. Changing the router ID under the OSPF process does not take effect until the OSPF process is explicitly restarted using 'clear ip ospf process' or the router is rebooted.
3
Evaluate the OSPF neighbor state recorded by R2 upon receiving R1's initial Hello packet
R2 transitions R1 to the Init state
When a router receives an OSPF Hello packet from a neighbor that does not contain its own Router ID in the Received Neighbor field, it places that neighbor into the Init state. MTU mismatches affect Database Description (DBD) exchanges during ExStart/Exchange states, not the initial Hello exchange.

Key Concept

OSPFv2 Router ID selection non-preemptiveness and finite state machine transition rules upon Hello packet reception.
Estimated Time:2m 0s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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