What is the primary purpose of hello packets in routing protocols?
Correct Answer: B
Detailed Explanation: Hello packets are a crucial element in many routing protocols. They are small messages exchanged between routers to establish and maintain neighbor relationships. These packets help ensure that adjacent routers are active and communicating, forming the basis for the exchange of routing information. How Hello Packets Work: When a router starts up or when it is configured with a routing protocol (such as OSPF, EIGRP, or ISIS), it begins to send hello packets out of its interfaces. These packets are transmitted at regular intervals. Each hello packet contains important information such as: The router’s identifier (e.g., router ID) The autonomous system number or process identifier Timers and parameters that help establish compatibility between routers A list of neighbor identifiers (in some protocols) Upon receiving a hello packet, a router checks the information to verify that the sender is running the same protocol with compatible settings. If the parameters match, the receiving router adds the sender to its neighbor table. This process continues as long as hello packets are received; if a router stops receiving hello packets from a neighbor (beyond a specified hold time), it considers the neighbor unreachable and adjusts its routing table accordingly. Significance in Routing Protocols: Hello packets are fundamental for: Neighbor Discovery: They allow routers to automatically identify which other routers are directly connected. This is the first step in building a complete routing topology. Maintaining Adjacencies: Continuous hello packet exchanges ensure that routers remain aware of their neighbors. Loss of hello packets indicates a potential link failure or misconfiguration. Synchronization: By exchanging hello packets, routers synchronize their routing information, which is essential for the convergence of the network. Comparison with Other Options: Option A (verify the integrity of data): While data integrity is important, hello packets are not primarily used for data integrity checks. Option C (calculate the shortest path): The calculation of the shortest path is performed by algorithms (e.g., Dijkstra for OSPF), not by hello packets. Option D (update the routing table): Although hello packets trigger the exchange of routing information, the actual update of the routing table is performed by the routing protocol’s update mechanisms after the neighbor relationship is established. Real-World Application: For example, in OSPF, hello packets are sent every 10 seconds (by default) to ensure that all routers on a network segment are still active. If a router fails to receive hello packets from a neighbor for a predetermined period, that neighbor is declared down, prompting a recalculation of routes. This mechanism minimizes downtime and ensures that the network quickly adapts to changes. Conclusion: The primary purpose of hello packets in routing protocols is to discover and maintain neighbor relationships (Option B). This process is essential for building and maintaining an accurate view of the network topology, which in turn allows for efficient routing and rapid convergence.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.