BackBone Fast Issue

[spalislam]

I am not sure if I should use backbone fast feature on my network design to save 20 sec of max-age time.

If I am designing daisy chain by switch A feeding switch B and switch B feeding switch C, can I use backbone fast in my connection from C to B in the case of failure of link A to B. Switch A is the root of the STP. Does backbone fast apply to the root port?

Thanks,

[liviu.gheorghe]

Backbonefast is a method to speed up convergence in a layer 2 environment in the event of an indirect link failure.

It applies to a root port or a blocked port and is triggered when it received inferior BPDU's from a designated bridge.

Here is a link that might help:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_7_5/confg_gd/stp_enha.htm#1020739

[spalislam]

Thank you very much sir,

I have read several articles about backbone fast. However, none of them mention my specific case. BBfast helps convergance of STP in event of an indirect link failure. However, my question is/was do I have any advantage of using it if my config is like this:

A switch (root)

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B switch (designated switch)

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C switch

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Client PCs

Now, if switch B losses its connection to the root (A), it will create an inferior BPDU and send it to the C. B will think it became the root. Now my question is will C ignore the B BPDU or it will accept it and since it will see it as inferior BPDU, it will cause Max age time to expire and announce itself as a root as well. So now I have two switches announcing to be root. Is the link between them still UP or it is unusable state. How many seconds are going to pass before they converge and choose the root? I have read a lot of papers, but none of them explain this particular situation in debth.

Once again thanks a lot. I would appreciate if you would be able to answer this question, so I can determine whether to use backbone fast in this situation.

[t.baranski · Accepted solution]

If your topology doesn't contain loops (which it doesn't appear to), you don't need to be worrying about Spanning Tree. BackboneFast won't help you here because Spanning Tree doesn't even help you here -- you have a loopless topology.