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Difficulty: MediumRouter Forwarding Decision Logic

A Cisco router receives an IPv4 packet on an ingress interface. Place the sequential steps of the router's forwarding decision and packet processing logic in the correct order from initial receipt to egress transmission.

  1. 1Inspect the packet's destination IPv4 address and search the routing table for matching network prefixes.
  2. 2Select the matching routing table entry that has the longest prefix match (most specific subnet mask).
  3. 3Extract the next-hop IPv4 address and designated outbound exit interface from the selected route entry.
  4. 4Query the ARP cache to resolve the Layer 2 destination MAC address associated with the next-hop IPv4 address.
  5. 5Decrement the IPv4 TTL field by 1, encapsulate the packet in a new Layer 2 frame header, and transmit the frame out the exit interface.

Answer

The correct sequence for router forwarding decision logic is: 1) Inspect the destination IPv4 address and search the routing table for matching prefixes, 2) Select the route with the longest prefix match, 3) Extract the next-hop IPv4 address and exit interface, 4) Query the ARP cache for the next-hop Layer 2 MAC address, and 5) Decrement packet TTL, encapsulate into a new Layer 2 frame, and transmit out the exit interface.
When a router receives an IPv4 packet, the forwarding process follows a strict sequence: First, the destination IPv4 address is compared against candidate routes in the routing table. Next, the router selects the route entry with the longest prefix match (most specific subnet mask). Once the best route is selected, the router identifies the next-hop IPv4 address and exit interface. It then queries the local ARP cache to find the corresponding destination MAC address for the next hop. Finally, it modifies the packet by decrementing the Time to Live (TTL) by 1, recalculating the IPv4 checksum, encapsulating the packet into a new Layer 2 frame with updated source/destination MAC addresses, and forwarding it out the exit interface.

Step-by-Step Solution

1
Receive packet and parse IPv4 header.
Destination IPv4 address extracted for routing table comparison.
The router must compare the destination address against candidate prefix entries in the routing table.
2
Apply Longest Prefix Match (LPM) evaluation.
The route entry with the highest mask length (e.g., /28 over /24) is selected.
LPM is the primary rule for forwarding lookup in Cisco routers.
3
Identify forwarding path details.
Next-hop IPv4 address and egress interface determined.
The matching route entry specifies the destination gateway and physical/logical outbound interface.
4
Perform Layer 2 resolution.
Target MAC address obtained from the ARP table.
Layer 2 framing requires the destination MAC address of the next-hop router or end-host.
5
Process packet header modifications and frame encapsulation.
Frame transmitted out the egress interface.
The router updates TTL/checksum fields and constructs new source/destination MAC addresses for outbound transmission.

Key Concept

Router Forwarding Decision Logic and Layer 2 Frame Rewrite
Estimated Time:1m 30s
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Topics

Router Forwarding Decision Logic

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