Spanning tree/Prof Messer

Quick question on this video: CompTIA Network+ N10-005: 1.4 - Spanning Tree Protocol | Professor Messer - CompTIA A+, Network+, Security+, Linux, Microsoft Technology Training

On the chart showing around 5:30 or so, some ports are marked BP for blocked port. These are opened to let traffic flow past the X which represents a link outage.

Why are these ports blocked in the first place? Is it to prevent loops? Also, how are they blocked? Does STP handle this?

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Comments

Adam B

If I'm not mistaken, STP Handles it, and it is to prevent loops, by not allowing traffic on that port

aftereffector

Yes. STP will automatically place certain ports in a blocking status to prevent routing loops. It does this by determining the best choice for each route and then blocking all other ports. The algorithm used to determine the 'best choice' is a little more complex to explain but for Net+, you shouldn't need to know anything about it other than that it exists.

When a link fails, STP will recalculate the 'best' routes and automatically enable some of the formerly blocked ports. This can take a little while - up to 50 seconds with default values for spanning tree.

ratbuddy

Thanks!

EdTheLad

aftereffector wrote: »

Yes. STP will automatically place certain ports in a blocking status to prevent routing loops. It does this by determining the best choice for each route and then blocking all other ports.
When a link fails, STP will recalculate the 'best' routes and automatically enable some of the formerly blocked ports. This can take a little while - up to 50 seconds with default values for spanning tree.

Routing loops refer to layer 3, better to say layer 2 loops, forwarding loops, switching loops etc

TheNewITGuy

Might want to look at this -> Cisco Is Easy: Lesson 19 - Spanning-Tree Protocol Overview

CoolAsAFan

Just started studying CCENT so take what I say with a grain of salt, but if it helps, STP is needed when you have redundant routes setup. If no STP and all ports were open and the target device was off/broken, broadcast traffic could get stuck in an infinite loop "searching" for the target device that is offline. This is why STP disabled certain ports, to prevent these forwarding loops. If a route fails, STP will auto open the next best route (ports) to prevent network outage.

TechGuru80

STP is automatic as soon as it detects redundant links but certain parts can be customized to make it more efficient. Rememeber that CompTIA exams are conceptual so the big idea is STP prevents layer 2 loops.