[Rstplib-users] Re: RSTP Ring Size ?

[Les B. <Les...@eu...>]

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
>
>
>