Spanning Tree root switch

Folks: I'm trying to understand importance of having a "high end" switch as the root switch. I see references of it here and there but haven't seen a solid document on it. I've spent quite some time googling spanning tree and root switches. I accidently ran into a situation where a low end "access" switch became the root and things still seem to be working and I'm trying to understand why and what the downsides are. Here is a little backgroud:

I have a fairly simple network, with a potential for growth and hence the need for proper STP configuratoin. I'm trying to understand the worse case scenario for a misconfigured "root" switch. I understand that I can force the root switch to be the core switch by changing the priority and also understand the use root gaurd protection and other techniques.

Here is what my network looks like:

Switch (M) Core switch and also desired STP root, VLAN 20,30,40 ) / | \\ switch A(VLAN 20) switchB (VLAN 20) switch C (VLAN 30)

Now it turns out that becuase of lower MAC ID, switch A become my root for VLAN 20.

Here is what my STP diagram looks like for VLAN 20.

Switch A (root) DP | RP Switch M (Core) DP | RP Switch B Questions:

  1. Things seem to be working with switch A as the root switch. Where should I look for spanning tree misconfiguration/bottlenecks? Any IOS command recommendations?
  2. What is the worse case scenario with having switch A as the STP root?
  3. Does the bottleneck only happen when spanning tree reconverges or would it affect layer 3 traffic in any way?


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John Carter
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Switch M: spanning-tree vlan 20 root primary

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I very much doubt that you need a "a "high end" switch as the root switch" however it is important that the root is at the centre of the network.

Think of the "tree" bit of "Spanning-tree". This name was not chosen randomly as many seem to be:-)

Imagine a squirrel on branch A of a fir tree wants to get to Branch B.

For most possible journeys in a tree the squirrel will head first of all towards the root then away from the root of the tree.

(Clearly there are exceptions for some nearby branches)

If your Root Bridge is on the network edge then there is the likelihood that a large proportion of your traffic will first head towards the network edge and then back towards the destination. This is not what most people want.

The worst case (and this is not at all unlikely) is that most network traffic goes via the root bridge (switch). This may for example mean that your gigabit core links have no traffic at all and that the uplinks to a particular "access" switch carry all of the traffic.

Get a network diagram and draw the Spanning tree on it and follow the traffic.

A trivial case which makes the point is two core switches say with Gigabit between them, and a single access switch connected by 100M to both core switches.

You have a server on one core switch and a tape-backup server on the other core switch. If the Access switch is ROOT then traffic between these two servers will go via the access switch. Probably not what is wanted.

Good luck.

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