[c-nsp] Limits of STP/RSTP/REP?

Garry gkg at gmx.de
Wed May 20 16:27:31 EDT 2009

Ross Vandegrift wrote:
> On Wed, May 20, 2009 at 08:09:17PM +0200, Garry wrote:
>> Wondering, what's the sensible limits of STP, RSTP, REP or any other
>> spanning tree/ring protocol available on Cisco switches like 29, 35, 37
>> or ME3400 series? I was told by a customer whom we try to sell some
>> Cisco gear that beyond anything like 4 or 5 switches in a ring,
>> recognition/recovery times of the ring would quickly go well beyond 10s
>> on failure of a link ...
>> Now, on STP the times are definitely somewhere in that range, but what
>> about RSTP or stuff like REP?
> In a usual ring scenario, an RSTP bridge would have two paths to root.
> For either cost or tiebreaker reasons, one would be chosen as the root
> port and one would be chose as the alternate.  If the root port goes
> down then the switch will rapidly move the alternate port to root and
> start sending BPDUs with the TCN flag set.  This will in turn cause
> the other bridges to age-out their MAC table.
> The process of flooding TCNs should lead to worst-case full
> reconvergence in approximately (hello-interval * max number of hops in
> the active topology) seconds.  For a ring of five switches, all of who
> have only edge ports on the non-ring interfaces, this is a worst case
> of six seconds until all bridges have flushed their MAC tables.
Question mainly is: Can Cisco gear handle a setup where there might be a
ring made of - say - 20-30 switches, each of which having two interfaces
each in the ring ("in" and "out", so to speak) ... while at the moment I
don't expect that customer to set up more than 4-6 switches to begin
with, locations are there that will require that number of switches over
time ... (sort of a MAN scenario)
To date, anything I've come across of (usually a set of 3-4 switches)
never caused any king of problem, but then very rarely were there any
line/link failures ...

Tnx, -garry

More information about the cisco-nsp mailing list