A Cisco router receives an IPv4 packet destined for host 192.168.50.45. The router's routi… — Cisco CCNA

A Cisco router receives an IPv4 packet destined for host $192.168.50.45$ . The router's routing table contains a static route `ip route 192.168.50.0 255.255.255.0 10.1.1.2` and a directly connected network $10.1.1.0/30$ out interface `GigabitEthernet0/1`. Arrange the sequential processing steps executed by the router to forward this packet from initial ingress lookup to egress transmission.

1Extract the destination IP address $192.168.50.45$ from the received packet header.
2Perform a routing table lookup for $192.168.50.45$ and identify the matching static prefix $192.168.50.0/24$ with next-hop IP $10.1.1.2$ .
3Perform a recursive lookup for next-hop IP $10.1.1.2$ to determine the outbound interface `GigabitEthernet0/1`.
4Query the ARP table (or send an ARP request) to resolve the Layer 2 destination MAC address for $10.1.1.2$ .
5Encapsulate the IPv4 packet into a Layer 2 frame with the target MAC address and transmit it out interface `GigabitEthernet0/1`.

Answer

The correct sequence is: Extract the destination IP address -> Perform primary routing lookup -> Perform recursive lookup for the next-hop IP -> Query ARP table for next-hop MAC -> Encapsulate packet and transmit out egress interface.

When a Cisco router receives an IP packet, it first extracts the destination IP address. Using longest prefix match, it matches the static route entry pointing to next-hop IP 10.1.1.2. Because this route specifies an IP next-hop rather than an outbound interface directly, the router performs a recursive lookup to find which connected interface reaches 10.1.1.2 (GigabitEthernet0/1). Knowing the egress Ethernet interface, it resolves the next-hop MAC address using ARP, encapsulates the IPv4 packet in an Ethernet frame, and transmits it out GigabitEthernet0/1.

Step-by-Step Solution

Inspect destination header

Destination IPv4 address

192.168.50.45

is extracted from the incoming frame.

Routing decisions are based on the packet's destination IPv4 address.

Execute primary route table lookup

Matches route `192.168.50.0 255.255.255.0` with next-hop IP

10.1.1.2

The router uses longest prefix matching to select the route entry.

Execute recursive lookup

Resolves

10.1.1.2

to directly connected route

10.1.1.0/30

on interface `GigabitEthernet0/1`.

Static routes configured solely with a next-hop IP address require a recursive lookup to identify the exit interface.

Resolve Layer 2 next-hop address

Obtains the target MAC address for

10.1.1.2

from the ARP table.

Ethernet encapsulation requires a destination Layer 2 MAC address for the next hop.

Encapsulate and egress

Frame is sent out `GigabitEthernet0/1`.

Final physical delivery step out the egress interface.

Key Concept

IPv4 Static Route Recursive Lookup & Packet Forwarding Pipeline

Estimated Time:2m 0s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic