[Rstplib-users] Re: RSTP Ring Size ?

[<rob...@ya...>]

Fine, but I don't understand, what is
a theoretical vindication of such
nonlinear effective age calculation? From where did
the magic "1/16" come?
Another question: when and for what should we
increase MaxAge more that 24 ?

Thanks, Robert

 --- Les Bell <Les...@eu...> wrote: > 
> 
> 
> 
> The 'problem' is caused by the way effective age of
> a BPDU is incremented at
> each Bridge, as it is received on its Root Port and
> subsequently propagated on
> its Designated Ports - see 802.1w 17.19.19 and
> 17.19.21.
> 
> "IThe effective age of the port information
> (portPriority and portTimes) is
> taken as the value of the Message Age parameter
> carried in a received BPDU,
> incremented by the greater of (1/16th Max Age) and 1
> s, and rounded to the
> nearest whole second."
> 
> This results in the effective age being incremented
> by 1 at each hop, for MaxAge
> <= 23; an increment of 2 at each hop for 24 <=
> MaxAge <= 39; and an increment of
> 3 at each hop for MaxAge == 40.
> 
> The network size is limited when the effective age
> of the received info from a
> BPDU exceeds MaxAge.  The received info must also be
> refreshed before it is aged
> and discarded, so the maximum network size is also
> affected by the value of
> HelloTime.  The limit of the network is reached when
> ((effectiveAge+HelloTime)>=MaxAge).
> 
> With default values of MaxAge (20) and HelloTime
> (2), the maximum network size
> 18 Bridge hops from the Root Bridge, giving a total
> of 37 Bridges in a chain or
> a ring, if the Root Bridge is at the centre. 
> Setting (MaxAge=24) causes an
> increment of 2 in root times at each hop, so the
> limit is reached after only 11
> hops from the Root Bridge, with either (HelloTime=1)
> or (HelloTime=2).  For
> (MaxAge=39) and (HelloTime=1), the limit is 19 hops
> from the Root Bridge.
> 
> The maximum achievable network size is with
> (MaxAge=23) and (HelloTime=1), with
> 22 hops from the Root Bridge giving a total of 45
> Bridges in a chain or a ring.
> In practice, you will probably have to reduce this
> to 21 hops from the Root
> Bridge (43 in a chain or a ring) to allow for timer
> variances between the
> Bridges, as you do not want the Bridges furthest
> from the Root Bridge to age out
> the information from its Root Port if the next BPDU
> is a few milliseconds late.
> 
> Les...
> 
> 
> 
> 
> 
> Robert Petroff <rob...@ya...> on
> 04/02/2002 11:58:05
> 
> Sent by:  Robert Petroff <rob...@ya...>
> 
> 
> To:   std...@ie...
> cc:   rst...@li... (Les
> Bell/GB/3Com)
> Subject:  Re: RSTP Ring Size ?
> 
> 
> 
> 
> 
> Would anyone like to ask on this (see below) my
> question, please ?
> I realy need it...
> Robert
> 
>  --- Robert Petroff <rob...@ya...>
> wrote:
> >
> > Hi,
> >
> > We hoped, that IEEE 802.1w (RSTP) will be able
> > to solve the problem of a big ring of Bridges.
> >
> > (In the our legacy STP (IEEE 802.1d) bridges we
> had
> > to "break" the spec.: we allow to exceed the value
> > 40
> > for MaxAge).
> >
> > From the first view, we saw, that a new protocol
> is
> > not so sensitive to the LAN diameter, because the
> > message age behaves much more like a hop count
> > in RSTP than it did in STP.
> >
> > But now, it seems to us, the state is worse :(
> >
> > I read the messages in your mailing list:
> > a) of Richa Malhotra on
> >
> http://www.ieee802.org/1/private/email/msg00221.html
> > with subject "802.1w -- Message/Max age"
> > b) of Alex Ruzin on
> >
> http://www.ieee802.org/1/private/email/msg00423.html
> > with subject "802.1w: How does LAN diameter depend
> > on MaxAge ?"
> >
> > It seems, that in IEEE 802.1w (RSTP) even our
> trick
> > with MaxAge will not help to increase LAN
> diameter.
> >
> > May anyone here provide with clarification, please
> ?
> >
> > I humbly thank you in advance, Robert
> >
> >
> >
> 
> 
> 
> 
> 
>