I’m currently studying for my CCNP SWITCH exam. Which is obviously all about switches, but mostly builds upon the foundation of the ICND2 and CCNA exam about switches and the Spanning Tree Protocol. So, if you are planning on getting your CCNP definitley make sure you pay attention to STP (Spanning Tree Protocol for short), you will need to know a lot about it.
Routers which operate at Layer 3 of the OSI model are able to prevent packets from looping around in the network forever because a packet only has a certain amount of time before it is expired. An expired packet is dropped or discarded and is completely removed from the network.
Frames Don’t Expire
Switches on the other hand have no mechanism for keeping frames from looping around the network for eternity, or until the switches are rebooted. Frames looping around the network can cause some serious problems. The main problem being that they can use up valuable bandwidth and slow down or even halt the network traffic completely. Not good.
It’s important and part of good network design to have redundant paths and switches, but with multiple paths only increase the switching loop problem and won’t make the network any faster. In order for these redundant paths between switches to exist the Spanning Tree Protocol or STP for short was created to fix these looping issues with switches.
The 5 STP Port States
There are five STP port states. This means that when STP is running on your switch it can take control of certain switch ports by blocking all frames that come through, or put it into listening mode where it listens to STP traffic. I explain these 5 steps in more detail below.
Blocking – Frames are not being forwarded.
Listening – In this state there are 3 different things that happen. (1) A root bridge gets selected (2) On the non-root bridge root ports get selected (3) Designated ports on the root bridge get selected. I will discuss these 3 STP steps in more detail below.
Learning – Captures MAC address and puts them in its MAC Address Table.
Forwarding – The switch is now forwarding frames and network traffic should be moving right a long now.
Disabled – This simply means that an administrator issued a ‘shutdown’ command on that switch interface. I wouldn’t even consider this part of STP, because there is nothing STP can do to bring up the interface. It has to manually be enabled again to be part of STP.
3 Steps in the STP Listening State
During the listening state of STP a root bridge will be selected. The root bridge is determined by the lowest Bridge ID and the lowest MAC Address. You will want to make sure this root bridge is in somewhat of a central location in your switching network.
After the root bridge has been selected STP will go to the non-root bridge and select specific port to be the root port. Also on the non-root bridge it will place the redundant port into blocking state so that loops don’t occur. The cool thing about STP is that when it detects that the root path has gone down it will automatically open up that blocked port so that traffic can then flow in the other direction keeping most of the network accessible.
A root bridge has been selected, and a non-root bridge has been chosen with a root port and a blocked port. Now whats left for STP to do is go back to the root bridge and select designated ports that will be the only interfaces that forwarding traffic will get sent on.
So, as a little recap some important key words to know with STP are, Root Bridge, Non-Root Bridge, Designated Ports, Root port, Blocking, Listening, Forwarding, and Disabled.
More to come…
There is a lot more to cover about STP which I will write about it my next post, plus I’m going to work on a short video explaining STP for all the visual learners out there.
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