RPVST and Switch Hop Limits

Attenuation
Attenuation used Ask the Experts™
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Hi All,

    I have a network that is already at about 9 switch-hops end-to-end.  We can't get around doing this, but knowing that STP recommends switch-hop limits to be 7, I've read that RSTP and RPVST(+) don't have this limitation.  Is this true?  I need to add a couple more switches.  It may not be optimal, but functional is more important right this second.  Bottom-line - does R-PVST have a switch hop limit like STP's 7?  We, obviously use R-PVST.

Thanks!
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Author

Commented:
To clarify - if you do know the answer - could you please provide some documentation to support or reference?  Thanks much.
Instructor
Top Expert 2015
Commented:
The diameter is not a limitation like the hop count in RIP. It is a method of determining effect on BPDU's through the network (Processing time, propagation delay, lost BPDU's, etc). It is also used to calculate the optimal timer values.

Switches have no way of knowing how many switches exist in a network. Make your network as big as you want and see if it works. Even with legacy (802.1d) spanning-tree, it will probably work fine with a diameter greater than 7.

Since in RSTP, each switch can generate a BPDU (as opposed to legacy STP where they are only forwarded) propagation delay is no longer a factor so you can have a larger network.

That said, if your switches suppport RSTP, you'll be better served by using it.
Don JohnstonInstructor
Top Expert 2015

Commented:
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Author

Commented:
Thanks for the response.  Are you suggesting, via this document, that I need to change the diameter settings?  The doc doesn't show the thought that delay concerns are not an issue with RSTP.   I feel better at this point about using the RPVST but still don't have that statement:)  Did I miss something?  I very much appreciate the help.
Don JohnstonInstructor
Top Expert 2015

Commented:
What changing the diameter does is change the timers (forward delay, max age, etc.). For example, if you change the diameter to 4, the forward delay timer will be automatically set to about 8 and max age to 10. If you set the diameter to greater than 7, spanning tree could reconverge even through there is no topology change.

This issue is effectively eliminated with RSTP.

If all my switches support RSTP, that's what I would use.

Author

Commented:
Situation worked just fine.  I very much appreciate the counsel and response!!!  There are very few search results that nail this.  To any concerned, my R-PVST+ ring is at least 13 Switches.  

Great Job - made my weekend better with confidence.

Thanks again

Author

Commented:
Oh hey - I also noticed in the STP stats that the topology change counters noted only local topolopgy changes.  The core, which has all VLANs leading back to it, never saw them flap around.  I presume this is by design. - Hinting that topology changes are only noticed by the switches they affect, despite being on the same VLAN...?
Don JohnstonInstructor
Top Expert 2015

Commented:
Yes, that's correct. No need to mess up the address tables of other switches if it doesn't affect them, right?

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