RSTP diameter calculation

Unanswered Question
Jan 12th, 2009

Is this formula (found googling on the web) correct to calculate the diameter (number of hops) on an RSTP Layer2 ring ?

MaxAge=(4x hello)+(2x diameter) -2

by default maxage=20, hello=2, then

the diameter would be 8 ...

Is this correct for RSTP ?

I have this problem too.
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Giuseppe Larosa Mon, 01/12/2009 - 05:59

Hello Gilberto,

a well designed and implemented RSTP network doesn't make use of maxage concept:

the more advanced BPDU handling should allow to make changes without waiting for max-age expiration:

on each point-to-point link a negotiation process named synchronizatio occurs and the two users will end to agree on port role and state.


However, you can use the old rules for more safety

Hope to help


grodoni Mon, 01/12/2009 - 06:10

Thanks Giuseppe.

Ok, but what happens on a ring with 20 switches or more when the root fails ? I saw an RSTP ring built with more than 20 non-Cisco industrial switches that has some troubles converging in case of failure of the root bridge.

There must be a "theoretical" and a "best of practice" limit in a RSTP ring topology....

Giuseppe Larosa Mon, 01/12/2009 - 13:20

Hello Gilberto,

if you have a ring topology and the root bridge fails : a new root bridge has to be elected.

With traditional 802.1D without max-age correct settings problem can arise.

With RSTP 802.1W this timer shouldn't play a role.


the document says that Cisco has implemented a proprietary version of the synchronization process: a simplified version where only the root port is object to handshake.

Also a switch is authorized to accept inferior BPDUs when it loses its connection to the Root Bridge.

As I wrote in my first post you can choice an appropriate value for max-age and other timers that reflects the topology.

if the root bridge fails the ring is broken and all devices have to elect a new root bridge.

Hope to help


grodoni Mon, 01/12/2009 - 23:31


thanks a lot for your answer,

I found the attache paper about the rstp convergence problems, which is also referenced in some other writings.

Giuseppe Larosa Wed, 01/14/2009 - 11:49

Hello Gilberto,

thanks for the link you have provided in your post.

Even if these are only simulations with a known network modeling tool they show the need for changing the max-age value.

Let us be practical increasing the max-age has no impact during normal and small failures.

Best Regards



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