How does a router know which protocol to use to reach a destination when multiple protocols are present?

• A. It uses Administrative Distance to choose the most trusted source, then the protocol metric
• B. It chooses the route with the shortest AS path
• C. It uses the newest learned route
• D. It floods all routes and lets the user decide

Answer: (D)

When a router sees routes to the same destination from different routing protocols, it uses Administrative Distance to decide which route to trust. Administrative Distance is a value that represents how trustworthy a route learned from a particular protocol is. The route from the protocol with the lower Administrative Distance is preferred and installed in the routing table, regardless of the protocol. If two routes have the same Administrative Distance, the router then compares the protocol’s metric (for example, cost in OSPF, hop count in RIP) to choose the best path. If those are also equal, other tie-breakers may apply. The idea isn’t to flood all routes and let a user decide; the router automatically makes this decision based on these built-in criteria.