spanning tree switched off

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Mar 29th, 2007
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Hi all, i have just found out on my network that spanning tree is turned off, we have 2 core routers, acting as gateways, they are running vrrp, so one is active, all access switches connect to both distribution switches, and the ditribution switches connect to each other, is there any reason why they would turn spanning tree off ?

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mahmoodmkl Thu, 03/29/2007 - 09:18
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Can u tell us where the spanning-tree is turned off.without full details of ur topology its not possible to give solutions.



carl_townshend Thu, 03/29/2007 - 12:33
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hi mahmood, they say it is turned off everywhere. why would they do that ?

Francois Tallet Thu, 03/29/2007 - 12:43
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Probably because they don't have redundancy at layer 2.


dgahm Thu, 03/29/2007 - 15:23
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Since you are able to post on NetPro it is a sure bet that you do not have redundant layer 2 links. A bridge loop with no spanning tree would kill your network pretty much instantly.

The reason to disable spanning tree is to improve link recovery time. With 802.1D spanning tree you will wait 50 seconds for a link to reach forwarding state. It also reduces the processor load on switches. The risk with disabling spanning tree is that someone will accidentally create a loop and meltdown the network.

Should you turn on STP? It depends on your topology, but if your access switch VLANs extend through the distribution switches to the core the answer is yes. If using layer 3 to the access layer, and your access switches are single chassis (or 3750 stacks) maybe not, though I would at least want to run it on user VLANs. Never underestimate the cabling creativity of a user with a hub he bought at CompUSA.

If you do enable STP be sure and do it during a maintenance window, as it will be disruptive.

Please rate helpful posts.


carl_townshend Thu, 03/29/2007 - 15:36
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thanks for your help, from what i can see the network consists of 2 layer 3 dist switches, each access switch connects to both dist switches, im sure they are running vrrp on the dist switches, but the dist switches are also connected to each other, so this would at least create a loop, unless its layer 3, but i cant see how it would be layer 3 between them if running vrrp

Francois Tallet Thu, 03/29/2007 - 15:54
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The L2 access switches can have no vlan in common. That means that for each vlan, the L2 topology is a V (no loop). The L3 boxes run vrrp through the access boxes on each vlan, not on the L3 link between them.

That's a possibility.

I think you should ask them for more information at that stage. All this is just guessing.



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