Question:
Which two circumstances can prevent two routers from establishing an OSPF neighbor adjacency? (Choose two.)
- mismatched autonomous system numbers
- an ACL blocking traffic from multicast address 224.0.0.10
- mismatched process IDs
- mismatched hello timers and dead timers
- use of the same router ID on both devices
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IFixYourWrongAnswers replied 1 year ago
The two circumstances that can prevent two routers from establishing an OSPF neighbor adjacency are:
D. Mismatched hello timers and dead timers: When two routers have different hello and dead timers, they may not be able to establish a neighbor adjacency. The hello timer specifies how often OSPF hello packets are sent to discover and maintain neighbor relationships. The dead timer specifies how long a router will wait before declaring a neighbor down if it doesn’t receive a hello packet.
B. An ACL blocking traffic from multicast address 224.0.0.10: OSPF uses multicast address 224.0.0.5 to send hello packets and multicast address 224.0.0.6 to send link-state advertisements (LSAs). If an ACL is blocking traffic from multicast address 224.0.0.10, the routers will not be able to discover each other and establish a neighbor adjacency.
The other options are not correct:
A. Mismatched autonomous system numbers: OSPF uses autonomous system numbers (ASNs) to identify OSPF domains. If two routers have different ASNs, they will not be able to form an OSPF neighbor relationship, but this would not prevent them from establishing a neighbor adjacency if they have the same ASN.
C. Mismatched process IDs: OSPF uses process IDs to differentiate between multiple instances of OSPF running on the same router. Two routers can form an OSPF neighbor relationship as long as they have the same process ID.
E. Use of the same router ID on both devices: OSPF uses the router ID to identify each router within an OSPF domain. If two routers have the same router ID, they will not be able to form an OSPF neighbor relationship, but this would not prevent them from establishing a neighbor adjacency if they have different router IDs.
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The two circumstances that can prevent two routers from establishing an OSPF neighbor adjacency are:
D. Mismatched hello timers and dead timers: When two routers have different hello and dead timers, they may not be able to establish a neighbor adjacency. The hello timer specifies how often OSPF hello packets are sent to discover and maintain neighbor relationships. The dead timer specifies how long a router will wait before declaring a neighbor down if it doesn’t receive a hello packet.
B. An ACL blocking traffic from multicast address 224.0.0.10: OSPF uses multicast address 224.0.0.5 to send hello packets and multicast address 224.0.0.6 to send link-state advertisements (LSAs). If an ACL is blocking traffic from multicast address 224.0.0.10, the routers will not be able to discover each other and establish a neighbor adjacency.
The other options are not correct:
A. Mismatched autonomous system numbers: OSPF uses autonomous system numbers (ASNs) to identify OSPF domains. If two routers have different ASNs, they will not be able to form an OSPF neighbor relationship, but this would not prevent them from establishing a neighbor adjacency if they have the same ASN.
C. Mismatched process IDs: OSPF uses process IDs to differentiate between multiple instances of OSPF running on the same router. Two routers can form an OSPF neighbor relationship as long as they have the same process ID.
E. Use of the same router ID on both devices: OSPF uses the router ID to identify each router within an OSPF domain. If two routers have the same router ID, they will not be able to form an OSPF neighbor relationship, but this would not prevent them from establishing a neighbor adjacency if they have different router IDs.