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STP Question

Posted on 2015-02-04
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Last Modified: 2015-02-07
Hi
Spanning tree Root Bridge for a vlan can never be behind a routed device (Layer 3)
Is this correct
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Question by:ciscosupp
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Don Johnston earned 300 total points
ID: 40589683
Not sure I understand what you're asking.

Spanning Tree BPDU's cannot pass through a router. So if you had two switches separated by a router, both switches would be a root bridge.
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by:ciscosupp
ID: 40589698
That’s what I was looking for thanks.

But you can configure a router to be spanning tree Root Bridge for a vlan.
Is this correct
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by:Don Johnston
ID: 40589719
That's a tough one...  A "router" is a layer-3 device that doesn't run STP.  

However, some routers have switch modules that you can install. Those switch modules are able to run STP.  

Also, on some routers you can disable layer-3 on a per interface basis at which point the interface is now a layer-2 interface and does participate in STP.  

But then you're not really talking about a router at that point because it's basically a switch (at least the interfaces in question are).
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by:Natty Greg
Natty Greg earned 50 total points
ID: 40589947
What you want is a switch 3550 is a good choice for that. No spanning tree behind a routed device.
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by:mikebernhardt
mikebernhardt earned 150 total points
ID: 40590054
You can take a router and make it bridge between 2 (or more) layer 3 interfaces, which means it isn't routing between those 2 interfaces. In that case it passes BPDUs and can participate in STP, or even be the root bridge.

If you use Integrated Routing and Bridging (IRB) You can even assign an IP address to a bridge interface so that it can route IP over those bridged physical interfaces. Strange, but true. We used to do this in order to let a router route from a T1 to a LAN through 2 different Layer 2 switches on the same VLAN.

I'm not saying this is the best design, just that in answer to your question it can definitely be done.
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