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.

Edit:

However, you can use the old rules for more safety

Hope to help

Giuseppe

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.

see

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

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

Giuseppe

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

Giuseppe,

thanks a lot for your answer,

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

Attachment: 
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

Giuseppe

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