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

Ross Vandegrift ross at kallisti.us
Wed May 20 19:34:05 EDT 2009

On Wed, May 20, 2009 at 10:27:31PM +0200, Garry wrote:
> 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)

Definitely not more than 20 in a ring.  As far as I know, IOS limits
the value of max-hops to 20.  This means you can't have a BPDU
traverse more than 20 hops without being thrown away.  If one pair of
switches in the ring experienced a total cut, your network would have
a diameter of 20, end to end.

JUNOS lets you set that value to 255, but I doubt that STP-like
protocols ever scale that well.  I don't know anything about the
various vendor-specific link redundancy features - my guess is you'll
have to go there.

Ross Vandegrift
ross at kallisti.us

"If the fight gets hot, the songs get hotter.  If the going gets tough,
the songs get tougher."
	--Woody Guthrie

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