Which two circumstances can prevent two routers from establishing an OSPF neighbor adjacency? (Choose two.)
Click on the arrows to vote for the correct answer
A. B. C. D. E.DE
OSPF (Open Shortest Path First) is a routing protocol that is commonly used in large enterprise networks to distribute routing information among routers. In order for two routers to establish an OSPF neighbor adjacency, they need to meet certain requirements. Let's look at the two circumstances that can prevent two routers from establishing an OSPF neighbor adjacency:
A. Mismatched Autonomous System Numbers: Autonomous System Number (ASN) is a unique identifier assigned to a network that uses the Border Gateway Protocol (BGP) for routing. When using OSPF, each router should be configured with the same ASN to establish an adjacency. If there is a mismatch in the ASN, the routers will not be able to establish an OSPF neighbor adjacency. This is because OSPF uses the ASN to determine which routers belong to the same network and which do not.
C. Mismatched Process IDs: In OSPF, a process ID is used to differentiate between different instances of the OSPF routing protocol on a single router. If two routers have different process IDs configured, they will not be able to establish an OSPF neighbor adjacency. This is because the process ID is used to identify the OSPF process running on each router and to distinguish between different instances of OSPF running on the same router.
B, D, and E are incorrect answers for this question because:
B. An ACL blocking traffic from multicast address 224.0.0.10: OSPF uses multicast address 224.0.0.5 to send OSPF hello packets to other routers in the network. An ACL blocking traffic from multicast address 224.0.0.10 will not prevent two routers from establishing an OSPF neighbor adjacency because OSPF does not use this multicast address.
D. Mismatched hello timers and dead timers: OSPF hello packets are used to establish and maintain neighbor relationships between routers. The hello packets contain information about the router, such as its router ID, priority, and OSPF interface state. The hello timer is the interval between two successive hello packets sent by a router. The dead timer is the interval after which a router declares a neighbor as dead if it does not receive a hello packet from the neighbor. If the hello timers and dead timers are mismatched between two routers, they will still be able to establish an OSPF neighbor adjacency, but it may take longer for the neighbor relationship to be established.
E. Use of the same router ID on both devices: Each router in an OSPF network must have a unique router ID. If two routers have the same router ID configured, they will not be able to establish an OSPF neighbor adjacency because OSPF uses the router ID to identify each router in the network.