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Difficulty: HardIPv4 Static Routing

A Cisco router receives an IPv4 packet destined for host 192.168.10.45192.168.10.45, which matches a static route configured strictly with a next-hop IP address (10.0.0.210.0.0.2) rather than an exit interface. Arrange the operational steps in the exact sequence the router executes to process, resolve, and forward this packet.

  1. 1Extract the destination IPv4 address 192.168.10.45192.168.10.45 from the incoming packet header.
  2. 2Search the IPv4 Routing Information Base (RIB) using Longest Prefix Match (LPM) logic.
  3. 3Identify the next-hop IPv4 address 10.0.0.210.0.0.2 associated with the matching static route.
  4. 4Perform a recursive lookup in the routing table to resolve next-hop 10.0.0.210.0.0.2 to an outbound exit interface.
  5. 5Consult the ARP table for the Layer 2 MAC address of 10.0.0.210.0.0.2, rewrite the frame header, and transmit the packet.

Answer

The correct sequence of router operation steps is: Extract the destination IPv4 address (192.168.10.45192.168.10.45), search the IPv4 RIB using Longest Prefix Match, identify the next-hop IPv4 address (10.0.0.210.0.0.2), perform a recursive lookup to resolve the exit interface, and consult the ARP table to encapsulate and transmit the packet.
When a Cisco router receives an IPv4 packet, it extracts the destination IP address from the packet header (192.168.10.45192.168.10.45). Next, it evaluates its routing table (RIB) using Longest Prefix Match (LPM) to find the most specific route entry. Upon finding the static route, it identifies that the destination is reached via next-hop IP 10.0.0.210.0.0.2. Because the static route was configured without specifying an egress interface, the router must perform a recursive lookup in the routing table to determine which directly connected interface reaches 10.0.0.210.0.0.2. Once the exit interface is resolved, the router queries its ARP cache to map 10.0.0.210.0.0.2 to a Layer 2 MAC address, re-encapsulates the packet into a new Layer 2 frame, and transmits it out the interface.

Step-by-Step Solution

1
Packet Ingress and Header Parsing
Destination IPv4 address 192.168.10.45192.168.10.45 is extracted.
The router needs the destination IPv4 address to perform a routing lookup.
2
RIB Query via Longest Prefix Match
The static route entry best matching 192.168.10.45192.168.10.45 is selected.
Routing decisions in Cisco IOS prioritize routes with the longest prefix length.
3
Next-Hop Extraction
Next-hop IPv4 address 10.0.0.210.0.0.2 is retrieved from the static route entry.
The router identifies that the traffic must be sent to 10.0.0.210.0.0.2 before reaching the final destination.
4
Recursive Route Resolution
Next-hop 10.0.0.210.0.0.2 is resolved to a directly connected exit interface.
Static routes configured solely with next-hop IP addresses require a second routing lookup to identify the physical outbound interface.
5
Layer 2 Resolution & Encapsulation
ARP table maps 10.0.0.210.0.0.2 to its destination MAC address, the packet is encapsulated into a Layer 2 frame, and forwarded.
The router must encapsulate the IP packet into a valid Ethernet frame using the exit interface and next-hop MAC address.

Key Concept

Recursive Static Route Lookup and Packet Forwarding Sequence
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Topics

IPv4 Static Routing

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