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Difficulty: MediumRouter Forwarding Decision Logic

When a Cisco router receives an IPv4 unicast packet and must select the single best route for forwarding from multiple potential routing table candidates, in what sequence does the router apply its decision logic? Place the steps in the correct order from first to last.

  1. 1Inspect the packet header to extract the destination IPv4 address.
  2. 2Filter candidate routes using Longest Prefix Match (most specific subnet mask length).
  3. 3Compare Administrative Distance (AD) to select the most believable routing source among identical prefix length matches.
  4. 4Compare routing protocol metrics to choose the lowest cost path among identical prefix matches from the same protocol.
  5. 5Determine the next-hop IPv4 address and associated outbound interface from the selected best route entry.

Answer

The correct order of router decision logic is: 1) Inspect the packet header to extract the destination IPv4 address, 2) Filter candidate routes using Longest Prefix Match (most specific subnet mask length), 3) Compare Administrative Distance (AD) to select the most believable routing source among identical prefix length matches, 4) Compare routing protocol metrics to choose the lowest cost path among identical prefix matches from the same protocol, and 5) Determine the next-hop IPv4 address and associated outbound interface from the selected best route entry.
Router forwarding decision logic follows a strict order of operations: First, the destination IPv4 address is read from the packet header. Second, the router searches its routing table for candidate routes and selects the longest prefix match (most specific mask). Third, if multiple routes have identical subnet mask lengths from different routing sources, Administrative Distance (AD) is evaluated to pick the lowest AD source. Fourth, if identical prefix routes come from the same routing source, the metric is evaluated to select the lowest cost path. Finally, the next-hop IP and egress interface are resolved for forwarding.

Step-by-Step Solution

1
Identify initial packet processing.
Destination IPv4 address is extracted from the IPv4 header.
The router needs the target IP address before searching the routing table.
2
Apply Longest Prefix Match (LPM).
Candidate routes are evaluated strictly by prefix length (number of matching subnet mask bits).
LPM is the primary criterion in IPv4 forwarding. AD and metric are ignored until LPM narrows candidates to identical prefix lengths.
3
Evaluate Administrative Distance (AD).
If a tie exists in prefix length between different routing sources, the route with the lowest AD is selected.
AD measures route believability/trustworthiness across different routing sources.
4
Evaluate protocol metric.
If candidate routes have identical prefix lengths and originate from the same routing protocol, the path with the lowest metric is chosen.
Metric measures the internal path cost for a specific routing protocol.
5
Extract forwarding parameters.
The router obtains the egress interface and next-hop IP address for packet encapsulation.
This information is needed to build the new Layer 2 frame and transmit the packet.

Key Concept

Router Forwarding Decision Hierarchy (LPM -> AD -> Metric -> Next-Hop Extraction)
Estimated Time:1m 30s
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Topics

Router Forwarding Decision Logic

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