Data Link Layer Switching

There are cases why multiple LANs may need to be joined.   There are also cases in which one LAN may need to be treated as multiple LANs. So in this article, I will talk about Joining LANs and Splitting LANs. 

Let's first discuss Joining LANs. The examples of networking devices to join networks are repeaters, hubs, and bridges/switches. They belong to the different layers of the TCP/IP stack. 

Repeaters are physical layer devices and do not understand frames/packet/headers. A signal appearing on one cable is amplified and put in another cable here. Hubs are also physical layer devices, and a hub has a number of input lines that it joins. Frames arriving on any line are forwarded to all other line. There is no signal amplification in a hub though, and it creates a single collision domain. 

A switch has a number of input lines and ports that it joins. Frames arrive on any line are forwarded to the line connected to the frame destination. Unlike a hub, each port is isolated to be its own collision domain. Common setup is a building with centralized wiring. The bridges are placed near the wiring closets. 

Here is the graph of the layer switching: 

Bridges can also be used as Layer 2 Devices. We use but don't remove ethernet header/addresses, and do not inspect Network Header. 

The bridge use requirements are plug-and-play, no hardware changes other than hooking LAN cables to bridge ports, no software changes, and transparency. No station should be aware there is a bridge in the middle. The bridge can know the port corresponding to each station by learning bridges. 

In Learning Bridges, the Learning Algorithm associates MAC address with bridge ports. Upon receiving a frame, we associate source address on frame with the input port. The output frame is associated with the frame's destination address. Unlearned destinations are sent to all other ports. These bridges need no configuration. The Spanning tree helps with the problem to prevent bridge topologies with looks to cause frames to circulate forever. 

The spanning tree bridges run a distributed algorithm to arrange themselves as nodes in a Minimum Spanning Tree. Bridges then need to elect a root for this minimum spanning tree, and the one with the smallest MAC address is used as root.  After the algorithm runs, here in the below diagram, B1 is the root, B4 uses link to B2, and B5 uses B3. 

Now let's discuss VLANs, or virtual LANs. The split one phyiscal LAN into multiple logical LANs to ease management tasks, reduce the scope of flooded frames, and improve security. 

Now let's discus In IEEE802.1Q, Bridges need to be aware of VLANs to support them Frames are tagged with their "color" and legacy switches/no tags are supported. 

Now Below shows a IEEE 802.1Q frame header, and this carries a color tag which is a VLAN identifier.