STP - Root Bridge Explanation Required

spicc7

Hey Guys,

Going for my CCNA in a few days.

I have been reading up on STP and I understand how the Root Bridge gets elected perfectly (i.e lowest priority and MAC address). What I can't seem to understand is why a Root Bridge is needed in the first place. I know it sounds simply, but I need a layman's explanation why we need a Root Bridge. Am I right in assuming the Root Bridge determines which ports on each switch becomes a designated port or is this decision made by each switch?

Thanks!

james_

Hi Spicc7,

STP operates by preventing loops and requires a Hierarchical tree structure, every tree grows from a root, in this case the root bridge. Without this structure I dont see how STP could prevent loops, it just wouldn't know which ports to block or forward, as the structure would just be chaotic.

In addition, think about traffic, ie multicast BPDUs. With one Bridge operating as the root, all BPDUs are originated from that bridge, and forwarded on by bridges further down the tree. Now think about what would happen if there was no root bridge? In effect all switches would think themselves the root bridge and would all multicast BPDUs every 2 seconds, which would beforwarded by all other switches along the way! That equals a LOT of traffic and potential slowdown of the network.

Oh, and after a LAN segement declares its Root Bridge, STP is responsible for calculating the lowest cost path from each switch to the Root bridge, this is how I understand it, I am sure someone will correct me if I am wrong!

Good luck with the CCNA!

James.

spicc7

James,

Thanks for your help - I re-read my STP study material. I missed some key stuff while reading, specifically "After a Root Bridge is decided upon, configuration BPDUs are sent only by the Root Bridge". With this information and explanation you gave, I was able to truly grasp the STP process.

Thanks again!