Administrative Distance vs. Metric — Why an IGRP Route Beats RIP in the RIB When a network is learned by both RIP and IGRP, why does the routing table install the IGRP route (flagged as IGRP) instead of the RIP route?

Introduction / Context:
Routers often learn the same destination via multiple routing protocols. The decision process first compares administrative distance (AD), a trust ranking per protocol, before considering metrics. Understanding AD explains why a route from one protocol wins over another in the routing information base (RIB).

Given Data / Assumptions:

• Destination is learned by RIP and IGRP simultaneously.
• Default AD values: RIP 120, IGRP 100 (lower is preferred).
• RIB selection: first by AD, then by metric within the same protocol.

Concept / Approach:
Administrative distance ranks source credibility. Since IGRP's AD (100) is lower than RIP's (120), the router installs the IGRP-learned route and discards the RIP route from the main table (though it may be kept as a backup depending on platform). Metric comparisons matter only within routes from the same protocol; cross-protocol selections never compare RIP hop count with IGRP composite metric.

Step-by-Step Solution:
List candidate routes grouped by protocol.Pick the group with the lowest AD (IGRP).Within that group, select the best metric (if multiple IGRP paths exist).Install the winning IGRP route in the RIB.

Verification / Alternative check:
Check platform defaults: show ip protocols shows AD; show ip route displays installed protocol codes and reasons. Changing AD (distance command) can alter selection order.

Why Other Options Are Wrong:

• A: Update timer frequency does not control RIB preference.
• C/D: Metrics are not compared across protocols; AD decides first.
• E: Coexistence is common; they can run together.

Common Pitfalls:
Assuming a “better metric” in RIP can override IGRP; it cannot if AD differs.

Final Answer:
IGRP has a lower administrative distance.