An edge router at a regional office receives an IPv4 packet destined for . The active routing table contains multiple valid routes from OSPF, BGP, and static configurations that match the destination network. Which TWO of the following statements correctly describe the decision process the router uses to select the forwarding path?
- The router evaluates and selects the route with the longest prefix match (most specific subnet mask) prior to considering administrative distance or metric.Answer
- BThe router inspects Layer 2 Frame Check Sequence (FCS) integrity values across candidate links to determine path preference before reviewing Layer 3 routing parameters.
- When multiple candidate routes have identical prefix lengths, the router selects the path associated with the lowest administrative distance value.Answer
- DWhen equal-cost candidate paths are present, the router relies on Port Address Translation (PAT) to map distinct source ports for outbound load balancing across interfaces.
Answer
The router first selects the route with the longest prefix match (most specific subnet mask). If candidate routes have identical prefix lengths, the router then selects the route with the lowest administrative distance.
When forwarding an IP packet, a router follows a strict decision hierarchy. First, it identifies the route with the longest prefix match (the most specific network mask). Second, if multiple routes from different sources have identical prefix lengths, the router compares their administrative distance (AD) and selects the route with the lowest AD value.
Step-by-Step Solution
Key Concept
Routing Decision Hierarchy: Longest Prefix Match and Administrative Distance