Spanning-tree is a protocol that runs on our switches that helps us to solve loops. Spanning-tree is one of the protocols that you must understand as a network engineer and you will encounter it for sure if you decide to face the Cisco CCNA R&S exam.
Non-root bridges need to find the shortest path to the root bridge. What will happen if we have a mix of different interface types like Ethernet, FastEthernet and Gigabit? Let’s find out!
Let’s start with a nice picture:
If you have played with some Cisco switches before you might have noticed that every time you plug in a cable the led above the interface was orange and after a while became green. What is happening at this moment is that spanning tree is determining the state of the interface.
This is what happens as soon as you plug in a cable:
Let me show you an example so I can explain a couple of things:
Portfast is a Cisco proprietary solution to deal with spanning-tree topology changes.
Portfast does two things for us:
Uplinkfast is a spanning-tree feature that was created to improve the convergence time. In this blog we’ll take a look at how uplinkfast works. Here’s the topology that we will use to demonstrate it:
Backbone Fast is used to recover from an indirect link failure. What does that mean? Let me show you an example of an indirect link failure and how spanning-tree deals with it:
Nowadays we see more and more routing in our networks. Routing protocols like OSPF and EIGRP are much faster than spanning-tree when they have to deal with changes in the network. To keep up with the speed of these routing protocols another flavor of spanning-tree was created…rapid spanning-tree.
This is the topology that we will use:
By default Cisco Catalyst Switches run PVST+ or Rapid PVST+ (Per VLAN Spanning Tree). This means that each VLAN is mapped to a single spanning tree instance. When you have 20 VLANs, it means there are 20 instances of spanning tree.
Is this a problem? Like always…it depends, let’s take a look at an example: