Cisco Certified Network Associate (CCNA) 200-301 · Free study guide

Objective 3.2 — Determine forwarding decisions

Forwarding is deterministic when the decision stages stay in order. Route installation chooses which candidates enter the table; packet forwarding chooses the most specific installed match for one destination.

Stage one: longest-prefix match

For a packet, identify every installed route whose fixed leading bits match the destination. The route with the greatest prefix length wins. A /32 host route beats a /24, which beats a /16, which beats /0. Administrative distance and metric do not let a shorter prefix override a longer one.

Build the habit of writing candidate prefixes before looking at preference numbers. This prevents a common exam error.

Stage two: administrative distance

Before forwarding, the router may have learned the same prefix from connected, static, OSPF, or other sources. Administrative distance expresses local trust in the source and helps decide which source's route is installed. Lower is preferred. A floating static deliberately uses a higher distance so a dynamic route remains preferred while available.

Distance is local to one router; it is not advertised as an end-to-end path quality and does not compare different prefix lengths.

Stage three: protocol metric

Within a routing protocol, metric selects among paths to the same prefix. OSPF uses cost. Another protocol may use a different metric with different meaning. Do not compare raw metrics across unrelated protocols. Equal-cost paths can be installed together when the platform and protocol allow it.

Recursive resolution

If the chosen route names a next-hop IP rather than a directly usable interface, the router performs another lookup for that next hop. Resolution must terminate in a connected path. A recursive chain that cannot resolve prevents forwarding even if the destination route looks correct.

Worked scenario

Routes include OSPF 10.0.0.0/8, static 10.20.0.0/16 with distance 200, and OSPF 10.20.30.0/24 with cost 50. Traffic to 10.20.30.9 uses the /24. Traffic to 10.20.8.9 uses the /16 despite its high distance because the /16 is more specific than /8. Distance 200 would matter only against another source for the same 10.20.0.0/16 prefix.

Verification

Query the exact destination if the platform supports it. Confirm the selected prefix, next hop, recursive route, neighbor resolution, and exit-interface state. Then test the return path; correct forward selection on one router does not prove bidirectional application reachability.

Common traps

• Mixing installation preference with packet matching.
• Comparing metrics from different protocols.
• Ignoring a more-specific host route.
• Assuming next-hop text proves recursion succeeds.
• Treating equal cost as packet-by-packet distribution for every flow.

Readiness checklist

• I always apply longest prefix first.
• I know when distance and metric are relevant.
• I can trace recursive lookup.
• I separate forward-path proof from end-to-end proof.