Thursday, January 17, 2008

Port Priority and Port Cost

Ok, I finally have it figured out.

Port Cost: The primary metric a switch uses to determine which port is root. The cumulative costs to root are used. The lower the better

Upstream router id: The secondary piece used to determine which port is root. The lower the better

Port priority: Sent to the downstream switch to tell it which port to select as root.

Port priority on comes into effect if port cost and neighbor id are equal. Which pretty much means it only matters if there are multiple uplinks going to the same switch and the port cost is equal. Then, to influence which port is root, one can set the port priority on the upstream switch.


Alex said...

Finally found this explanation, very useful! Cheers.

Anonymous said...

Great explanation. Probably best one around