From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Sun Nov 02 2008 - 10:22:56 ARST
Rob,
this is a complicated question, since it depends on the "type" of failure
and engaged STP features, such as uplink fast and backbone fast. Provided
that any of these special feature are disabled, consider the following
situations:
1) Active uplink fail and there is working alternate port. Since the
alternate port keeps receiving the superior BPDUs, it will take 2xFwdTime to
transition the port to forwarding state. (With UplinkFast this is almost
instant).
2) Active uplink stops receiving configuration BPDUs, but the switch
considers the port to be up. This may happen if there is a failure in some
link not directly connected to the bridge. In this situation, the switch
will wait maximum for an MaxAge time (usually less, since every switch on
the downstream path decrements this counter) to expire the old information
before accepting any new superior/inferior BPDUs. This is the situation
which Backbone fast was designed to counter (avoid waiting MaxAge time of
some other switch signal about network failure via inferior BPDU). In this
scenario, converge will take at maximum MaxAge + 2xFwdTime or 50 seconds by
default.
Based on all this, it is hard to predict the exact convergence time in
"general" case. However, based on the Occam's razor principle, I would
suggest considering just the simple case of direct link failure, since it
involves tuning just a single parameter, which is FwdTime. Besides, playing
with MaxAge may lead to serious instabilities in STP topology (e.g. disjoint
STP portions) faster than playing with FwdTime ;)
However, always pay attention and try to see if the task is actually asking
for Uplink/Backbone Fast features.
-- Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice) petr@internetworkexpert.comInternetwork Expert, Inc. http://www.InternetworkExpert.com Toll Free: 877-224-8987 Outside US: 775-826-4344
2008/11/2 Rob Clav <robclav@gmail.com>
> Hi Guys, > after reviewing cisco docs, and groupstudy old mails regarding > forwarding time It's not clear for me yet. > From my point of view convergence time for pvst+ is the time between > blocking state to forwarding state (50 seconds by default), so max age > should be changed as well. > But there are several answers from people: > > 1st solution, the most extended one says, total time 8 seconds? > spanning-tree vlan 1 forward-time 4 (that value is the minimum) > > 2on solution, total time 14 seconds??? > spanning-tree vlan 1 forward-time 4 > spanning-tree vlan 1 maxage 6 > > The second one for me is the convergence complete time. what do you think? > > The non Cisco links are: > http://cciepursuit.wordpress.com/2007/08/12/spanning-tree-timers-reminders/ > http://www.groupstudy.com/archives/ccielab/200606/msg01446.html > http://www.groupstudy.com/archives/ccielab/200504/threads.html#01150 > http://www.groupstudy.com/archives/ccielab/200504/msg01150.html > http://www.groupstudy.com/archives/ccielab/200510/msg01570.html > > > Blogs and organic groups at http://www.ccie.net > > _______________________________________________________________________ > Subscription information may be found at: > http://www.groupstudy.com/list/CCIELab.html
Blogs and organic groups at http://www.ccie.net
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