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. This worst case isn't very realistic. 802.1D-2004 specifies that a bridge should immediately transmit a BPDU if it has new information, including topololgy changes. It also specifies that there is an absolute max of 1.0 seconds permitted between an external event requiring a BPDU and the transmission of that BPDU. That gives a more realistic worst-case of three seconds. [1] In practice, I've seen tons of link failures happen with no measurable impact on traffic. That doesn't mean there wasn't any, but it's quick enough that I don't usually drop a ping during link maintenance. Of course this isn't a super-fine measuring tool, so do your own testing! Cisco has a decent article on various features and changes in RSTP that you might find interesting: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml And finally, if that isn't enough, the IEEE lets you download 802 specs for free at http://standards.ieee.org/getieee802/. 802.1D-2004 (and .1Q too) is actually pretty readable for someone serious about understanding it. [1] Of course, it's anyone guess as to how close the vendors come to implementing this :). -- Ross Vandegrift [email protected] "If the fight gets hot, the songs get hotter. If the going gets tough, the songs get tougher." --Woody Guthrie _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
