When two Layer 3 routers need to exchange traffic between each other, the process involves several steps to ensure that the packets are correctly routed from one router to the other. Let’s walk through the example scenario where Laptop #1, with IP address 192.168.1.1/24, is connected to the first router, and Laptop #2, with IP address 192.168.2.1/30, is connected to the second router. Laptop #1 wants to send a packet to Laptop #2.

  1. Source and Destination IP Address:
    • Laptop #1, with IP address 192.168.1.1/24, initiates a communication with Laptop #2, which has IP address 192.168.2.1/30.
    • Laptop #1 determines that Laptop #2 is on a different network based on the subnet masks.
  2. Routing Table and Next Hop Selection:
    • The first router examines its routing table to determine the next hop for the packet destined to 192.168.2.1/30.
    • Since static routing is used, the first router has a manually configured entry that specifies the next hop as the IP address of the second router’s interface connected to the first router.
    • The first router determines that the next hop for Laptop #2’s IP address is the second router.
  3. Encapsulation and Packet Forwarding:
    • Laptop #1 encapsulates the IP packet with source IP address 192.168.1.1 and destination IP address 192.168.2.1 in a Layer 2 frame, typically Ethernet.
    • The first router receives the frame from Laptop #1 and examines the destination IP address.
    • Based on its routing table, the first router determines that the packet should be forwarded to the second router.
    • The first router encapsulates the packet in a new frame and sends it out of the appropriate interface connected to the second router.
  4. Inter-Router Link and Forwarding the Packet:
    • The frame carrying the packet travels through the inter-router link, typically a physical or logical connection between the first and second routers.
    • The second router receives the frame and examines the destination IP address.
    • The second router looks up its routing table and determines that the packet’s destination IP address is directly connected to its interface with the IP address 192.168.2.1/30.
  5. Delivering the Packet:
    • The second router decapsulates the frame and forwards the original IP packet to Laptop #2 based on its destination IP address.
    • Laptop #2 receives the packet, extracts the encapsulated data with source IP address 192.168.1.1 and destination IP address 192.168.2.1, and processes it.

In this way, the two Layer 3 routers facilitate the exchange of traffic between Laptop #1, with IP address 192.168.1.1/24, and Laptop #2, with IP address 192.168.2.1/30. The routers use their routing tables and static routes to determine the appropriate next hop for the packet, encapsulate the packet in Layer 2 frames, and forward it through the inter-router link until it reaches its destination. The IP addresses of the laptops and the routing table entries enable the routers to route the packets correctly and establish end-to-end communication.