Cisco CCNA (200-301)IP ConnectivityHard
A router's routing table contains the following entries for the destination network 192.168.1.0: `O 192.168.1.0/24 [110/10] via 10.0.0.1` `S 192.168.1.0/26 [1/0] via 10.0.0.2` Which route will the router choose to forward a packet destined for 192.168.1.10?
- AThe router will load balance between both routes.
- BNeither, as there is a conflict in the routing table.
- C`S 192.168.1.0/26 [1/0] via 10.0.0.2`
- D`O 192.168.1.0/24 [110/10] via 10.0.0.1`
Show answer & explanationAnswer & explanation
Correct answer: C. `S 192.168.1.0/26 [1/0] via 10.0.0.2`
The router will use the longest prefix match rule. 192.168.1.0/26 is a more specific route (longer prefix) than 192.168.1.0/24. Even though the static route has a higher administrative distance (1) than OSPF (110), the longest prefix match takes precedence over administrative distance when selecting a route for a specific destination IP address.
Why the other options are wrong
- A. Load balancing typically occurs with equal-cost paths, and the longest prefix match determines which route is used, not load balancing between different prefixes.
- B. There is no conflict; the longest prefix match rule resolves such scenarios.
- D. This route is less specific (/24) than the static route (/26), so it will not be chosen for 192.168.1.10.
Longest Prefix Match Rule
When a router receives a packet, it compares the destination IP address with all routes in its routing table and selects the route with the longest (most specific) matching prefix.
- Takes precedence over Administrative Distance and metric.
- A /26 prefix is more specific than a /24 prefix.
- Ensures traffic is sent to the most precise known destination.
- Fundamental principle of IP routing.
Memory trick: Longest Prefix is the first choice, then AD shouts its voice, then Metric's rejoice.