An enterprise border router maintains multiple dynamic routing protocols alongside static routes to reach remote subnets. When an incoming IPv4 packet arrives at an ingress interface, the router's decision process evaluates several criteria to select the winning route entry. In what sequential order does the router evaluate these routing criteria to select the path for packet forwarding?
- 1Filter the routing table to identify all candidate route entries that match the packet's destination IP address.
- 2Select the candidate route with the longest prefix length (most specific subnet mask).
- 3Compare the Administrative Distance (AD) values if multiple routes have identical prefix lengths from different protocol sources.
- 4Compare protocol-specific metric values if multiple candidate routes originate from the same routing protocol with identical prefix lengths.
Answer
The correct order of route selection evaluation is: 1) Filter for matching candidate routes, 2) Apply Longest Prefix Match, 3) Compare Administrative Distance for equal-length prefixes, and 4) Compare protocol metric values for ties within the same protocol.
Router decision engines process route selection in a strict hierarchy: First, candidate routes matching the destination IP are gathered. Next, Longest Prefix Match (LPM) selects the route with the most specific prefix length. If candidate routes have identical prefix lengths from different routing sources, Administrative Distance (AD) determines trustworthiness. Finally, if routes have equal prefix lengths and originate from the same routing protocol, the protocol's internal metric determines the best path.
Step-by-Step Solution
Key Concept
Routing Table Selection Hierarchy (Longest Prefix Match -> Administrative Distance -> Metric)