Examkin

Soru

Zorluk: OrtaOSPFv2 Neighbor Adjacencies and Router ID

Two Cisco routers, R10 and R20, are directly connected via their GigabitEthernet0/1 interfaces. OSPFv2 is configured on both routers, but running the command 'show ip ospf neighbor' on R10 reveals no neighbor entries. IP ping between the directly connected interface addresses succeeds. Which interface configuration difference between R10 and R20 will prevent the routers from even forming an OSPF Init state?

  1. Mismatched OSPF Area IDs on the interconnecting interfacesCevap
  2. B
    Mismatched Interface MTU settings on the interconnecting interfaces
  3. C
    Mismatched OSPF Process IDs on the routers
  4. D
    Mismatched DR Priority values on the interconnecting interfaces

Cevap

Mismatched OSPF Area IDs on the interconnecting interfaces prevent the routers from establishing an OSPF neighbor relationship.
OSPF routers require several parameters to match in incoming Hello packets before creating a neighbor entry. Because Area ID is part of the Hello header validation, a mismatch causes the router to drop the Hello packet, resulting in no neighbor entry (no Init state).

Adım Adım Çözüm

1
Analyze OSPF Hello packet primary validation requirements.
For two OSPFv2 routers to establish a neighbor relationship, fields in the Hello packet must match across the link: Area ID, Subnet Mask, Hello/Dead Timers, and Authentication credentials.
If any primary parameter such as Area ID or Subnet Mask mismatches, the router ignores the incoming Hello packet entirely.
2
Evaluate the impact of an Area ID mismatch.
When R10 receives a Hello packet with a different Area ID than what is configured on its interface, it discards the packet, preventing any neighbor entry from appearing in 'show ip ospf neighbor'.
Area membership must strictly match for interfaces sharing a common network segment.
3
Contrast with MTU, Process ID, and DR Priority behavior.
Process IDs are local; DR priorities affect election roles only; MTU mismatches allow neighbor discovery up to ExStart state.
Distinguishing between parameters that break Hello discovery versus parameters that stall database synchronization is key to OSPF troubleshooting.

Anahtar Kavram

OSPFv2 Neighbor Adjacency Requirements and Hello Packet Matching
Bu soruyu puanla

Konular

OSPFv2 Neighbor Adjacencies and Router ID

Bu konudan daha fazla soru