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I have seen spanning tree being configured on switches to prevent switching loops.

I recently noticed that show spanning tree command works on a Cisco 1841 router as well.

For what reasons would we want to configure spanning-tree on router. Any examples?

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It's there because there are switch modules for the platform -- (H)WIC-4ESW. There's very little point to running STP on a routed interface. –  Ricky Beam Apr 14 at 19:01

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up vote 5 down vote accepted

On a general note: not everything that is present in parser, will actually work on given platform, or even makes sense.

In that particular case, you can enable spanning tree if you have either IRB (integrated routing & bridging) configured on ethernet ports, or you insert the integrated switching module (WIC-4ESW) into the router. Then, those ports are L2 by default and actually it makes sense to run STP on them to avoid loops.

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If you have

no split-horizon

you should have spanning tree enabled. but that command is nearly never used.

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Are you 100% sure? Can you provide a reference? –  radtrentasei Apr 15 at 19:48
    
No I am not 100% sure. –  Xiuhtecuhtli Apr 16 at 21:22

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