diameter of the RSTP (IEEE 802.1w-2001)

[tarhan_a]

Maximum bridge diameter

is 7 in according with RECOMMENDATIONS of the STP (802.1D).

Is there a recommendation for the Rapid STP at all?

[ankurbhasin]

HI Tarhan,

RSTP also recommends to have a diameter of 7

http://www.ruggedcom.com/pdfs/white_%20papers/Rapid_Spanning_Tree_in_Industrial_Networks.pdf

http://www.automation.com/sitepages/pid1338.php

Regards,

Ankur

[tarhan_a]

Hi Ankur,

thanks a lot for your feedback.

Frankly, we do have a stupid situation:

the customer is already running RSTP ring

based on 8x Catalyst and gonna expand

this ring till 11 switches...

Surely, there are ring solutions with > 7 switches.

Look at http://www.wwp.com/solutions/whitepapers/Spanning-Tree-Whitepaper.pdf

especially, Table 'Performance - Ring Size'.

Moreover, in document from your 1st link it was stated in conclusions "...Practical rings should be limited to 31 switches. A useful rule of thumb is to budget 3 milliseconds of recovery time for every switch in the ring.

..."

However, all above infos were special solutions. So, I'd be glad to find out about

"standard Cataylst" solutions.

the latest Cisco Document "keeps secret"

the ring size ;-)

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

Thank you again!

Rgds

[Francois Tallet]

RSTP uses the BPDU age as a hop count. So you can go beyond the (already conservative) diameter of seven in an RSTP network. If your network is RSTP only, you can basically configure max-age a little bit larger than your diameter with no problem. There is no impact on convergence time in a network that is adapted to RSTP as RSTP does not use any timers in favorable cases (RSTP falls back on timers in very few occasions). You can even reduce the value for max-age if you have a small network (the worst case scenario for RSTP is when the root bridge dies, in which case the root information may disappear with a typical distance vector count to infinity. Tuning max-age will prevent old root information to circulate for too long in the network).

Regards,

Francois

[tarhan_a]

Hi Francois

Thanks for your feedback.

http://www.cisco.com/en/US/customer/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml#awkward

As mentioned before we do need "an official recommendation" for a RING Topology based on RSTP. The default max-age=20s should not be

interpreted as a possible ring with 40 bridges!?

Do you have additional informations redarding to the "count to infinity". What is the equal with 16 RIP hops ;-)

Thanks & Regards