Two Cisco routers, R1 and R2, are connected via a direct GigabitEthernet link. R1 has been configured with an explicit OSPF router ID of 1.1.1.1. Router R2 has no explicit router ID configured under its OSPF process, but has the following active interfaces:
- Loopback0: 10.10.10.1/32 (up/up)
- Loopback1: 10.20.20.1/32 (up/up)
- GigabitEthernet0/0: 172.16.1.1/24 (up/up)
Which two conditions or behaviors accurately reflect the OSPFv2 Router ID selection and neighbor adjacency formation process between R1 and R2? (Select two.)
- Router R2 automatically selects 10.20.20.1 as its OSPF router ID because it is the highest IP address among active loopback interfaces.Answer
- Hello intervals, Dead intervals, Area IDs, and subnet masks must match between R1 and R2 for a neighbor relationship to form across the link.Answer
- CRouter R2 selects 172.16.1.1 as its router ID because active physical interface IP addresses take precedence over virtual loopback interfaces.
- DAn MTU mismatch between R1 and R2 will block Hello packet exchange entirely, keeping the neighbor state permanently in DOWN.
Answer
The correct statements are that R2 automatically selects 10.20.20.1 as its router ID (highest IP address among active loopbacks), and that Hello intervals, Dead intervals, Area IDs, and subnet masks must match between the neighbors to establish adjacency.
Router ID selection follows a strict order: explicit 'router-id' command > highest active loopback IP > highest active physical interface IP. Since R2 has active loopback interfaces, 10.20.20.1 is selected as the highest loopback IP. Additionally, OSPF neighbor formation strictly requires matching Hello/Dead timers, Area ID, Subnet Mask, and Authentication flags in exchanged Hello packets.
Step-by-Step Solution
Key Concept
OSPFv2 Router ID Precedence and Hello Packet Neighbor Adjacency Requirements
Estimated Time:2m 0s