CCNA IP Routing Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  Extract the destination IP address from the packet.

  Why this is correct

  The process of IP routing begins by examining the Layer 3 header of the incoming frame after decapsulation. The destination IP address is the key identifier that the router uses to make a forwarding decision; without extracting and interpreting this field, the router cannot consult its forwarding table. This step is the prerequisite for all subsequent longest-prefix-match and route-selection logic.

• ✓

  Search the routing table for all entries that match the destination IP address.

  Why this is correct

  Once the destination IP address is known, the router performs a lookup in its routing table (or Forwarding Information Base) to find every prefix entry that contains that address within its range. This is not a single exact-match lookup; rather, the router compares the address against all route prefixes to determine which ones are applicable. The set of matching routes forms the candidate list for the best-route selection process.

• ✓

  Identify the route(s) with the longest prefix length (most specific match).

  Why this is correct

  Among the candidate routes that match the destination address, the router selects the one with the longest prefix length, i.e., the most specific match. A /32 host route is preferred over a /24, a /24 over a /16, and so on, because longer prefixes represent a more precise path to the destination. This rule ensures that the most granular routing information available in the table dictates the forwarding decision.

• ✓

  If multiple routes share the same prefix length, compare their administrative distances.

  Why this is correct

  When two or more routes have the same prefix length and both cover the destination IP, the router must break the tie by comparing their administrative distances (AD). Administrative distance is a Cisco-assigned trustworthiness rating for the source of the route, such as connected (0), static (1), or EIGRP (90). Lower AD values indicate a more reliable routing source, so the route with the smaller AD wins at this stage.

• ✓

  Select the route with the lowest administrative distance among them.

  Why this is correct

  The administrative distance comparison resolves the tie by selecting the route whose originating protocol or mechanism is considered most trustworthy. For example, a static route with AD 1 is preferred over an OSPF route with AD 110 when both have the same prefix length. This decision does not involve metrics; it purely reflects the router's confidence in the routing source itself.

• ✓

  If multiple routes have the same administrative distance, compare their metrics.

  Why this is correct

  If multiple routes remain tied after the administrative distance comparison—for instance, two OSPF routes with equal AD—the router proceeds to compare their metrics. Metrics are protocol-specific values that quantify the cost or distance of a path, such as OSPF cost, EIGRP composite metric, or RIP hop count. Only routes from the same routing protocol can have their metrics meaningfully compared; different protocols use incompatible metric scales.

• ✓

  Choose the route with the lowest metric as the best route.

  Why this is correct

  The final tie-breaker selects the route with the lowest metric value as the best route to install in the routing table. For OSPF, this means the lowest cumulative cost; for EIGRP, the lowest composite metric derived from bandwidth and delay, among other factors. This step completes the route-selection process, and the winning route is then used to forward the packet to the next hop.