How Spanning-Tree Works Network Fundamentals Part 26

As we saw in the last video, spanning-tree prevents loops at layer 2. To do this, it first needs to find the loops.
It does this by assigning each port a cost. This is based on the speed of the port. The faster the port, the lower the cost.
As BPDUs are sent from the root bridge through the network, the cost accumulates. Each switch then know which ports it can use to reach the root bridge, and the cost of using each port.
If it can reach the root bridge on more than one port, then there is a loop that needs to be handled.
The lowest cost port becomes the root port. This is the port closest to the root-bridge. Other valid ports become designated ports. The root bridge cannot have root ports, so all its ports are designated.
Alternate paths to the root bridge represent loops, so they are moved to the blocking state. Therefore, we can see that root port face toward the root bridge, and designated ports face away.
It takes time to work all this out, so each switch goes through phases of blocking, listening, learning, and then forwarding. Regular data is only allowed to pass when the port is in the forwarding state.
This has some drawbacks, which has lead to the development of spanning-tree improvements, such as pvst, rstp, rpvst, and mst. Some of these are Cisco only, and some are open to all vendors.
Overview of this video:
0:00 Introduction
0:21 Spanning-Tree Topology
2:08 Blocking
5:32 Topology Changes
6:53 Port States
8:52 Flavours of Spanning-Tree
Quiz Link: https://networkdirection.net/labsandquizzes/quizzes/spanning-tree-quiz
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