OSPF Hello Packet Matching Fields
Question
You are asked to deploy OSPF between two routers over a broadcast medium.
Which three fields in the OSPF hello packet must match? (Choose three.)
Answers
Explanations
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OSPF (Open Shortest Path First) is a popular link-state routing protocol used for routing IP packets within a single autonomous system (AS). In OSPF, routers exchange hello packets with their neighbors to establish and maintain neighbor relationships.
When OSPF is deployed over a broadcast medium (e.g., Ethernet), each router on the network segment must agree on several parameters to form an adjacency. Three fields in the OSPF hello packet must match between routers for neighbor relationships to form:
Hello interval: This field specifies how often the router sends hello packets. The hello interval must match between routers to ensure that they hear each other's hellos and establish a neighbor relationship.
Options: This field specifies various OSPF options and capabilities, such as authentication and flooding scope. The options field must match between routers for neighbor relationships to form.
Network mask: This field specifies the subnet mask of the network segment. The network mask must match between routers for neighbor relationships to form.
Two additional fields in the OSPF hello packet relate to the election of the designated router (DR) and backup designated router (BDR):
Designated router: This field specifies the router that serves as the designated router (DR) for the network segment. The DR is responsible for sending LSAs (Link State Advertisements) on behalf of the network segment. If a DR is not elected, all routers on the segment must send LSAs, leading to increased flooding and processing overhead. The DR field does not need to match between routers.
Backup designated router: This field specifies the router that serves as the backup designated router (BDR) for the network segment. The BDR takes over as the DR if the current DR fails. Like the DR field, the BDR field does not need to match between routers.
In summary, when deploying OSPF between two routers over a broadcast medium, the three fields in the OSPF hello packet that must match are: hello interval, options, and network mask.
