Layer2 -Maximum size with Spanning-tree

Brett Martin · ‎06-24-2015

Hello,

I have attached a network topology diagram showing the existing Layer2 network (ring with single spur), and the proposed expansion (spur extends to additional ring). There will be a grand total of 20 switches.

My concern is Rapid PVST+ and the amount of switch nodes. Are there any limitations or best practices to follow for designing capacity for these networks--I cannot seem to locate anything.

We are in a bit of a pickle as we plan to interconnect the 2 Layer 2 networks in a week or so.

If anyone can chime in and help me out I really appreciate it!

Joseph W. Doherty · ‎06-24-2015

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Posting

Yea, STP has certain "expectations" concerning the topology's diameter; but I don't recall what all they are. You should be able to find out more by searching for STP and diameter. I recall, though, you can expand diameter if you adjust STP timers.

Brett Martin · ‎06-24-2015

Hi Joseph, you seem to have hit the nail on the head! I have found some of this information out there but nothing to confirm or identify what the maximum amount of hops from root can be supported.

Do you or others have any recommended links or documents that would cover this in enough detail to support this proposed design?

Thanks!

InayathUlla Sharieff · ‎06-24-2015

The STP standard specifies the max diameter with 7 and the default max age with 20 seconds (with 1 second delay per switch).

Joseph W. Doherty · ‎06-24-2015

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

That sounds somewhat familiar. If I also remember correctly, the diameter is the maximum span of hops across the topology, not just hops from the root bridge.

I believe you can often easily run larger diameters, using defaults, because of the 1 second default allowance, per hop. Switches have gotten a bit faster since STP was originally defined.

PS:

I broke down, and did a quick search and found: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/19120-122.html and http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html.