Think your layer 3 routing skills will be enough to get you through your career, not to mention to a passing mark on the CCNA? Think again.

So long as Layer 2 bridges, switches and VLANs are around, you'll need to know more than you probably want to about the Spanning Tree Protocol STP. The norm, throughout your career, will be a network constantly in transition, a mix of layer 2 bridges and switches with layer 3 routers and perhaps a Layer 4 device or two. That and STP is on the test, to make a mental note: learn all about STP.

To that end, let us begin.

Spanning Tree is one of three bridging methods a network administrator can use. Which method you use usually will be determined by the network’s size. The simplest method is transparent bridging, where only one bridge or switch exists on the network. The next is Source-Route, in which bridging address tables are stored on each PC on the network. Then there’s what you came for, spanning tree, which prevents loops where there exists more than one path between segments.

For a nice breakdown of Transparent vs Source-Route, click here.

Which brings us to the Spanning Tree Protocol. STP is a bridge/switch protocol that uses the Spanning Tree Algorithm (STA) to avoid or eliminate loops on a layer 2 network (remember, routers word at Layer 3 so when you're talking STP, you're talking Layer 2, bridges and switches). The STA enables a learning bridge or switch to dynamically work around loops in a network's topology. These loops are created by the network itself. If the devices that connect the network segments are all configured to forward, they will continuously forward frames into an endless network loop. Get enough loops going and a frame will not reach its destination.

STP deals with this loop formation by enabling only one active path at a time. Bridges and switches exchange BPDU messages with other bridges and switches to detect loops and then remove them by shutting down selected bridge interfaces. One of the protocol's key benefits is in fault tolerance because there are redundant paths to fall back on if the primary path should fail.

All of that, in a nutshell, is what STP is. But, obviously, there's more to it than that. We'll start with what confuses a lot of techs, that algorithm.

Spanning Tree Algorithm (STA)

Y'know, when I first was learning basic networking concepts, my eyes used to glaze over whenever I saw words like "algorithm." Well, Spanning Tree Algorithm (STA) is not a difficult concept. Just bear with me.

First of all, an algorithm is nothing more than a set of steps taken to solve a problem. A recipe for vegetable soup is an algorithm. So, now that you know that part isn't complicated. . .

Digital Equipment Corporation developed STA to preserve the advantages of loops while eliminating the disadvantages. Digital's version of STA was latre revised by the IEEE 802 committee and was published in the IEEE 802.1d specification. Please note, Digital's algorithm and the IEEE 802.1d algorithm are not compatible and that all Cisco switches run on the IEEE 802.1d version

If an algorithm is a set of steps taken to solve a problem, then you know Spanning Tree Algorithm is a set of instructions designed to solve a problem on the network. That problem STA solves is those loops on the network. STA deals with this problem by designating a loop-free subset on the network's topology. It does this by placing those bridge ports that, if active, would otherwise create loops, into blocking. Blocking bridge ports can always be activated in the event of a primary link failure by providing a new path through the network.

Forward or block or . . .

Blocking is just one of a number of port states into which a port can be placed. In STP, ports can be set for blocking, disabled, forwarding, learning and listening. Switch ports can transition between these states. Here's how they break down:

Blocking - Port accepts BDPUs but doesn't act on or forward them. By default, all ports are in this state when the bridge or switch is turned on. Ports with...

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