On a multiaccess broadcast network, which parameter is most influential in determining the Designated Router (DR) election in OSPF?
Correct Answer: D
Detailed Explanation: Open Shortest Path First (OSPF) is a link-state routing protocol that is widely used in enterprise networks. In multiaccess broadcast networks (such as Ethernet), OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR) to reduce the number of adjacencies and limit the amount of routing protocol traffic exchanged on the network. The DR serves as a central point for exchanging link-state advertisements (LSAs) with all other routers on the segment. The DR election process is influenced primarily by the OSPF priority setting on each router’s interface. This priority is a configurable value (ranging from 0 to 255) that directly affects a router’s chance of becoming the DR. A higher OSPF priority increases the likelihood that a router will be elected as the DR. If two or more routers have the same priority, the router with the highest Router ID is selected as the DR. How the DR Election Works: Priority Check: Each router sends hello packets containing its OSPF priority. The router with the highest priority on the network segment is elected as the DR. Backup Designated Router (BDR): The router with the second-highest priority becomes the BDR. The BDR monitors the DR and takes over if the DR fails. Tie-Breaker: If multiple routers share the same priority, the election falls back to comparing Router IDs. The router with the highest Router ID is chosen as the DR. Hello Packet Exchange: The election process occurs during the exchange of hello packets, which also serve to maintain neighbor relationships. Significance of OSPF Priority: By adjusting the OSPF priority on interfaces, network administrators can control which router becomes the DR. This control is important because the DR handles a significant portion of the routing protocol’s overhead. In some cases, it may be desirable to have a router with more processing power or a more reliable connection serve as the DR. Administrators can set the OSPF priority accordingly to influence the election outcome. Other Factors: Router ID: While the Router ID is used as a tie-breaker, it is not the primary factor if priorities differ. Interface Cost: This parameter influences the calculation of the shortest path but does not affect the DR election process. Hello Interval: The hello interval is related to the timing of neighbor discovery and is not a factor in the DR election. Practical Considerations: It is common for network administrators to explicitly set the OSPF priority on interfaces where the DR role should be predictable. For example, on a broadcast segment with multiple routers, the most robust router might be assigned a high priority (e.g., 255), while all others are assigned lower priorities. This configuration helps ensure that the DR remains stable and that the network converges efficiently. Conclusion: The most influential parameter in determining the OSPF Designated Router on a broadcast network is the OSPF priority (Option D). Mastery of this concept is critical for effective OSPF design and troubleshooting in multiaccess environments.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.