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Difficulty: HardIPv4 Static Routing

A Cisco IOS router receives an IPv4 packet destined for host 192.168.2.45192.168.2.45. The router's routing table contains a static route configured as `ip route 192.168.2.0 255.255.255.0 10.1.1.2`. Place the processing steps the router executes to resolve the exit interface and encapsulate the packet in the correct sequential order from first to last.

  1. 1Match the packet's destination IP address against the longest prefix in the IPv4 routing table to select the static route.
  2. 2Identify that the static route points to next-hop IPv4 address 10.1.1.2 rather than an outbound physical interface.
  3. 3Perform a recursive lookup in the routing table to locate an active route matching next-hop address 10.1.1.2.
  4. 4Match the directly connected route 10.1.1.0/24 to identify egress interface GigabitEthernet0/1.
  5. 5Retrieve the Layer 2 MAC address for 10.1.1.2 from the ARP table to encapsulate and transmit the frame out GigabitEthernet0/1.

Answer

The correct sequential order for recursive static route lookup and forwarding is: 1) Match destination IP against the routing table to select the static route; 2) Identify next-hop IP address 10.1.1.2; 3) Perform a recursive lookup in the routing table for 10.1.1.2; 4) Match the connected route 10.1.1.0/24 to determine exit interface GigabitEthernet0/1; 5) Retrieve the ARP table entry for 10.1.1.2 to encapsulate and transmit the frame.
When a static route is configured using only a next-hop IP address (e.g., `ip route 192.168.2.0 255.255.255.0 10.1.1.2`), Cisco IOS must execute a recursive lookup process. The router first matches the packet destination IP (192.168.2.45192.168.2.45) against the static route in the routing table. Second, it identifies the configured next-hop IP (10.1.1.210.1.1.2). Third, it performs a secondary (recursive) lookup for 10.1.1.210.1.1.2 in the routing table. Fourth, it finds the directly connected route (10.1.1.0/2410.1.1.0/24), revealing the physical egress interface (GigabitEthernet0/1). Fifth, it queries the ARP table for the MAC address belonging to 10.1.1.210.1.1.2 to build the Ethernet frame and transmit it out GigabitEthernet0/1.

Step-by-Step Solution

1
Evaluate the incoming packet's destination IPv4 address.
The router matches host destination 192.168.2.45192.168.2.45 to static prefix 192.168.2.0/24192.168.2.0/24.
Routing decisions start by determining the longest matching prefix for the destination IP address.
2
Inspect forwarding parameters specified in the matched static route entry.
The router determines that the route specifies next-hop IP address 10.1.1.210.1.1.2.
Static routes configured strictly with a next-hop IP address do not directly name an exit interface.
3
Execute a recursive lookup for IP address 10.1.1.210.1.1.2.
The router searches the RIB for a route covering 10.1.1.210.1.1.2.
Recursive resolution is mandatory when a next-hop IP is used without an explicit exit interface specification.
4
Resolve the next-hop IP to a connected network route.
The router matches 10.1.1.210.1.1.2 to connected network 10.1.1.0/2410.1.1.0/24 on interface GigabitEthernet0/1.
The connected route provides the physical exit interface necessary to transmit the traffic.
5
Query the ARP table and perform Layer 2 frame encapsulation.
The destination MAC address associated with 10.1.1.210.1.1.2 is acquired and the frame is transmitted via GigabitEthernet0/1.
Multi-access Ethernet media require resolving the next-hop IPv4 address to its target MAC address before physical transmission.

Key Concept

Recursive Routing Lookup for IPv4 Static Routes
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Topics

IPv4 Static Routing

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