Networking Primer – Part 6.3: Layer 2 Switching – Loops, Spanning Tree and Topologies

Previous: Networking Primer – Part 6.2: Media Access Control – CSMA/CD, CSMA/CA

We were briefly introduced to devices called Network Switches in the last post in this series. A switch essentially acts as a central connection point in a star topology  for many network nodes.It is similar to a Hub from a topological perspective but whereas a hub will take a frame in from one port and broadcast it out on all of the other ports, a switch has some built-in intelligence so it may forward the frame only to those ports which should receive the frame.  I like to think of a switch very much like it’s similar namesake, the switchboard, from the public telephony world.

Old Telephony Switchboard

In this older world, you picked up your phone to call the operator.  When the operator at the other end answered, you would tell her/him who you would like to call, they would cross reference the name with the relevant port number on the switchboard and plug in a cross-connecting wire between your incoming port to your outgoing call recipients. A network switch operates in a similar fashion although there are of course some notable differences.

Switch Ports

Ports are a very important entity in the switching process. Modern switches can contain 8, 16, 24 .. or even 1000′s of ports in large-scale enterprise level implementations. Port occupancy on a network switch can be very transient with desktops and laptops changing the port they are plugged into on a daily basis. To cope with this, the switch must be much more malleable and must have a mechanism for learning which device is occupying which port. It does this by maintaining a table of the source MAC addresses it receives from each port.  It is worth being aware that if a switch doesn’t know which port of the destination MAC address it will still broadcast to all the other ports in the same way a hub does.

I can’t emphasise the following enough, so it is worth re-iterating.. the Port is a very important entity in the switching process and is not only a node’s physical access point into the network.  It also represents a management construct that can be used to control the nodes security and resource permissions within the network.  The Port and it’s associated ID can be used to segment traffic as well as shape it (e,g, restrict bandwidth, etc).

Switching Topologies

A single switch device connecting all the nodes in a network is a pretty simple architecture to visualise and understand. This kind of set-up is however only found in small office environments. In larger environments, it may become impossible to cable all of the nodes into the same switch due to geographical, redundancy or resiliency factors.  In these environments, we need to introduce multiple interconnected switches.  Luckily most modern switches have the intelligence to connect to other switches in pretty much any configuration. We can daisy chain them together, make circular loops or any other artistic creation we wish.. all of these are possible:

Logical Topologies

When a switch is connected to another switch, it soon learns that the interconnecting port isn’t occupied by a single node and MAC address. They’ll learn that there is another switch there and that the port is possibly the destination for many devices. Any source MAC addresses coming in from that port will be stored in the table so that local nodes may send frames back to those devices via that port.  Given this flexibility, of connecting switches together in any configuration, it is possible to find ourselves with the problem of circular switching loops.

Switching Loops and the Spanning Tree Protocol

As stated above, if a switch receives a frame on a port and hasn’t yet learned the forwarding port of its MAC Address, it will broadcast it out on all of its ports with the exception of the one it receives it from. This is called a broadcast of an unknown unicast frame. A similar bulk multi-port forwarding operation may occur for general broadcast frames as well as multicast frames (frames for more than one destination node).  These multi-port broadcast have the potential to turn into infinite circular loops where there is a circular route to follow in an architecture.

Take the following example:

Switching Loops

A node connected to Switch B wants to communicate with a node connected to Switch C. It doesn’t know where the forwarding port for this node is so Switch B broadcasts to all ports including the ports interconnecting A, C, D & E. Switch A will send it to C, D & E. The frame will reach its destination on Switch C, but it may receive two copies of the frame, one from B one from A. Also now that D is in the mix, it’s possible D could broadcast it back to B, who in turn will broadcast it back to A. This is just one example of a switching loop.

The problem with these loops is that they’re often difficult to spot. The frame does get where it’s going, but multiple copies of it are being looped. This is only really apparent when the switches CPU seems to be increasing workload for no apparent reason. Enter STP or Spanning Tree Protocol. In Brief, STP learns the multiple possible routes a frame may take across the switching infrastructure. It then assesses these multiple routes using an algorithm to select the best one and blocks the rest, thus preventing any looping.

The Hierarchical Network Model

The Hierarchical Network Model is a network design model created by Cisco. It’s a very simple layered model created from medium or large network environments. The Layers are defined as follows:

  • Core Layer – composed of powerful high throughput switches and border routers to make up the backbone of the network
  • Distribution layer – a second tier layer used for aggregation the lower layer switches and connecting through to core.
  • Access Layer – the tier containing the front-end switches where the network devices/nodes gain access to the network.

The Hierarchy Network Model


This model is very widely deployed and has become somewhat of a defacto standard. It is worth remembering these layers and where they sit. In future blogs, I intend to address network virtualization and how it has shifted the dotted line and pulled some of the access layer into the hypervisor (more to come on this later).

 Next: Part 6.4: VLANs and other ANs (Area Networks)

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