Which will finally get the response frame back to Host A. The blue switch then consults its MAC address table to determine that the frame destined to should be forwarded out port 1. Therefore, the blue switch will learn the MAC address exists out port 3. The response frame then arrives on the blue switch on port 3. The green switch then consults its MAC address table to determine that the frame destined to should be forwarded out port 4. Therefore, the green switch will learn that the MAC address exists out port 6. The response frame will first arrive on the green switch on port 6. In the response frame sent by Host B to Host A, the Layer2 header will have a Source MAC address of and a Destination MAC address of. Each switch learned the location independent of the other there was no communication between the switches or sharing of MAC address tables. Specifically, both switches know the location of the MAC address – port 1 on the blue switch and port 4 on the green switch. The switches have already learned about some of the connected devices, and that should alleviate some of the additional flooding that was required for the initial communication in the previous section. On the way back things will go a little simpler. Host B will receive the frame and accept it for processing, since Host B was the intended destination. Host D will receive the frame, and silently discard it since the frame was not addressed to Host D. This behavior prevents a switch from sending a frame out the same port it was received. This is an example of a switch’s filtering behavior. Notice in both cases, the frame was flooded out each port, except the port it was received on. And again, just like the other switch, the green switch does not know where the MAC address exists, so the frame will again be duplicated and flooded out each switch port. Just like the other switch, the first thing the green switch will do is learn that it received a frame on port 4 with a source MAC address of. The frame will also arrive on the green switch. The frame arrives on Host C, who will inspect the frame and realize it is not the intended recipient. Then, since the blue switch does not yet have an entry in tis MAC address table for, the frame is duplicated and flooded out every port. When the frame arrives on the blue switch, the first thing that happens is the blue switch learns the MAC address exists on port 1. They will populate as the switches learn of each device connected to each port by reading the Source MAC address field of each received frame. To begin, the MAC address tables for both switches will be empty. The switches will use the information in the Layer2 header to move the frame between the two hosts. The Layer2 header would include a Source MAC address of and a Destination MAC address of. The Layer3 header would include a Source IP address of 10.0.0.11 (Host A) and a Destination IP address of 10.0.0.22 (Host B). The contents of the frame are irrelevant, it could be an ICMP (ping) packet, it could be an ARP packet, or it could be other data. It starts with Host A having a frame to deliver to Host B. Continued communication between Host A and Host B.We will illustrate each step that occurs for each of the following: Port 3 on the blue switch is connected to port 4 on the green switch. Host B and D are connected to ports 5 and 6 of the green switch on the right. Host A and C are connected to ports 1 and 2 of the blue switch on the left.
Our topology has two switches, and each has their own, independent MAC address table - the information in the MAC address tables are never shared. We will illustrate how data moves between multiple switches using the following topology: The process does not change, it is simply replicated separately by other switches. With multiple switches, each switch will still independently perform the exact same four functions. In the Packet Traveling series, we discussed the four specific functions of a switch: Learning, Flooding, Forwarding, and Filtering we then illustrated each function in an animation that showed two hosts communicating through a single switch. This article will illustrate the process using two switches.
One of the most frequent requests I receive is asking how the process would be different if there were multiple switches. In a previous article we demonstrated how a single switch functions.