From: Darby Weaver (ccie.weaver@gmail.com)
Date: Sun Nov 02 2008 - 11:09:01 ARST
Exactly.
Be careful of the wording of the task and the interpretation that you
employ:
change the "total convergence time" versus say... change the listening or
learning or time or change the "total forwarding time"... And as Petr
mentioned be mindful of other features in usage
recall that total convergence time by default is 50 seconds and takes
max-age into consideration to complete the equation.
So this is truly one of those "interpretation" questions that requires due
dilegence and "troubleshooting" that CCIE #1024 is famous for.
On Sun, Nov 2, 2008 at 7:22 AM, Petr Lapukhov
<petr@internetworkexpert.com>wrote:
> 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.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com <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
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
Blogs and organic groups at http://www.ccie.net
This archive was generated by hypermail 2.1.4 : Mon Dec 01 2008 - 08:18:28 ARST