When two Layer 2 switches need to exchange traffic between each other, the process involves several steps to ensure that the packets are correctly forwarded from one switch 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 switch, and Laptop #2, with IP address 192.168.1.2/24, is connected to the second switch. Laptop #1 wants to send a packet to Laptop #2.

  1. Source Address Resolution Protocol (ARP):
    • Laptop #1, with IP address 192.168.1.1/24, initiates an ARP request to determine the MAC address of Laptop #2 with IP address 192.168.1.2/24.
    • The ARP request is broadcasted on the local network, including the first switch.
    • The first switch receives the broadcast and examines its MAC address table to see if it has Laptop #2’s MAC address.
  2. MAC Address Learning:
    • If the first switch does not have Laptop #2’s MAC address in its table, it floods the ARP request out to all its ports, including the port connected to Laptop #2.
    • The second switch, to which Laptop #2 is connected, receives the ARP request and updates its MAC address table, associating Laptop #2’s MAC address with the port connected to Laptop #2.
    • Laptop #2 responds to the ARP request with its MAC address, and the second switch updates its MAC address table with this information.
  3. Forwarding the Packet:
    • Now that the first switch knows the MAC address of Laptop #2, it can forward the packet.
    • Laptop #1 encapsulates the IP packet with source IP address 192.168.1.1 and destination IP address 192.168.1.2 in an Ethernet frame with the destination MAC address set to Laptop #2’s MAC address and the source MAC address set to Laptop #1’s MAC address.
    • Laptop #1 sends the frame to the first switch.
    • The first switch looks up the MAC address table and determines that Laptop #2’s MAC address is associated with the port connected to the second switch.
    • The first switch forwards the frame out of the appropriate port toward the second switch.
  4. Inter-Switch Link (ISL) or Trunk Port:
    • The frame travels through the inter-switch link (ISL) or trunk port that connects the first switch to the second switch.
    • The ISL carries traffic from multiple VLANs, allowing VLAN-tagged frames to pass between the switches.
  5. Second Switch:
    • The second switch receives the frame on its trunk port.
    • It examines the destination MAC address and checks its MAC address table.
    • Finding a match, the second switch determines the port associated with Laptop #2’s MAC address and forwards the frame accordingly.
  6. Delivering the Packet:
    • The frame is delivered to Laptop #2, which has IP address 192.168.1.2/24, based on its MAC address.
    • Laptop #2 receives the frame, extracts the encapsulated IP packet with source IP address 192.168.1.1 and destination IP address 192.168.1.2, and processes it.

In this way, the two Layer 2 switches facilitate the exchange of traffic between Laptop #1, with IP address 192.168.1.1/24, and Laptop #2, with IP address 192.168.1.2/24. The switches use their MAC address tables to determine the appropriate ports to forward the frames, and the IP addresses enable the laptops to communicate at the network.