A Cisco router receives an IPv4 packet destined for host 172.16.5.45. The routing table co… — Cisco CCNA

A Cisco router receives an IPv4 packet destined for host $172.16.5.45$ . The routing table contains a static route configured with a next-hop IP address: `ip route 172.16.5.0 255.255.255.0 10.0.0.2`. In what chronological order does the router execute the forwarding decision and recursive lookup process to transmit this packet? Place the steps in order from first to last.

1Extract the destination IPv4 address ( $172.16.5.45$ ) from the incoming packet header.
2Perform a routing table lookup to find the longest matching prefix for $172.16.5.45$ , matching the static route $172.16.5.0/24$ .
3Perform a recursive lookup for next-hop address $10.0.0.2$ to resolve the directly connected egress interface.
4Query the ARP cache for $10.0.0.2$ to determine the Layer 2 destination MAC address, encapsulate the packet into an Ethernet frame, and transmit it.

Answer

The correct operational order begins with inspecting the packet header to extract the destination IPv4 address, followed by executing a longest prefix match in the routing table. Next, because the matching static route points to a next-hop IP rather than an exit interface, the router performs a recursive lookup to find the egress interface. Finally, the router resolves the next-hop MAC address via ARP, encapsulates the packet into a Layer 2 frame, and transmits it.

Packet processing follows a strict sequential pipeline in Cisco IOS: Layer 3 packet destination extraction → Longest Prefix Match routing table lookup → Recursive resolution of the next-hop IP address to an exit interface → ARP MAC address resolution and Layer 2 frame transmission.

Step-by-Step Solution

Packet Ingress & Header Inspection

Destination IP address

172.16.5.45

is extracted.

Routing decisions require identifying the target host IP in the IP packet header.

Primary Routing Table Lookup

The static route entry `172.16.5.0/24 via 10.0.0.2` is selected based on Longest Prefix Match.

Cisco IOS routes traffic using the most specific matching network mask in the routing table.

Recursive Next-Hop Lookup

The next-hop IP

10.0.0.2

is resolved to a directly connected network and outbound interface.

Static routes configured solely with a next-hop IP require an additional lookup step to identify which physical or logical interface sends the traffic.

Layer 2 Encapsulation & Transmission

ARP table yields the destination MAC address for

10.0.0.2

, frame is rewritten and transmitted.

Multi-access interfaces (like Ethernet) need a specific destination Layer 2 MAC address to transmit the packet.

Key Concept

IPv4 Static Route Recursive Lookup & Packet Forwarding Logic

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