Re: Forward-delay time

From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Wed Aug 30 2006 - 15:11:02 ART

Hi guys,

Probably it has to do with "correct" STP timers computations. A nice
description
is here:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml

Although in a lab, one may change timers carelessly, just to make "show"
commands
output look like they want it to look :)

2006/8/30, Brian McGahan <bmcgahan@internetworkexpert.com>:
>
> Leandro,
>
> What is your reasoning behind max-age coming into play?
>
> Brian McGahan, CCIE #8593
> bmcgahan@internetworkexpert.com
>
> Internetwork Expert, Inc.
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>
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of
> > ROCHA Leandro ORANGE-FT
> > Sent: Wednesday, August 30, 2006 12:13 PM
> > To: ccielab@groupstudy.com
> > Subject: RE: Forward-delay time
> >
> > I see. This question is a little different the one I was thinking
> about.
> >
> > I previously saw some similar question who explicitly said: "The
> entire
> > time from blocking to forward should be less than 16 secs for ports on
> > vlan X. Config the switches to accomplish this without jumping any
> state"
> >
> > This question avoids using portfast (don't skipping states), leading
> you
> > to have to tune max-age and fwd-delay to minimum. Is there any other
> > solution?
> >
> > Leandro
> >
> >
> >
> >
> >
> > "Scott Morris" <swm@emanon.com>
> > Sent by: nobody@groupstudy.com
> > 08/30/06 01:33 PM
> > Please respond to "Scott Morris"
> >
> >
> > To: "'Sean C'" <Upp_and_Upp@hotmail.com>, ROCHA Leandro
> > ORANGE-FT
> > <leandro.rocha@orange-ft.com>, "'Aamir Aziz'" <aamiraz77@gmail.com>
> > cc: "'GroupStudy'" <ccielab@groupstudy.com>
> > bcc:
> > Subject: RE: Forward-delay time
> >
> >
> >
> > While technically your thinking is correct and a good idea in REAL
> LIFE
> > network design, in the CCIE lab you are often presented with scenarios
> > that
> > logically would not play in real life.
> >
> > Don't over-think things! With a two-switch ethernet network, there
> really
> > isn't any magical difference that setting the max-age timer is going
> to
> > play! Watch your "show spanning-tree" information to see what the
> > switches
> > do.
> >
> > Although the lab scenario may ask you to specifically change one
> piece,
> > don't automatically assume you have to recalculate everything else!
> If
> > you
> > aren't asked to, don't do it.
> >
> > If the scenario asked you to keep all of the 802.1D ratios, yet still
> make
> > convergence time faster with those specifics, THEN perhaps you'll
> think
> > down
> > this path!
> >
> > Believe me, this is a path that many people take because of real-world
> > expectations that we have and knowledge of the actual
> > application/implication of changing things like this. But in a lab
> > environment, particularly with the limited pieces of equipment that we
> > have,
> > you should ask yourself whether it will make a difference, or not work
> > properly otherwise.
> >
> > HTH,
> >
> >
> > Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
> JNCIE
> > #153, CISSP, et al.
> > CCSI/JNCI-M/JNCI-J
> > IPExpert VP - Curriculum Development
> > IPExpert Sr. Technical Instructor
> > smorris@ipexpert.com
> > http://www.ipexpert.com
> >
> >
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of
> > Sean
> > C
> > Sent: Wednesday, August 30, 2006 11:49 AM
> > To: ROCHA Leandro ORANGE-FT; Aamir Aziz
> > Cc: GroupStudy
> > Subject: Re: Forward-delay time
> >
> > Hi Leandro,
> >
> > If the task was asking to go from blocking to forwarding state in
> under 16
> > secs, wouldn't that be when you would take into consideration the
> max-age
> > timer?
> >
> > This task specifically states to alter the forward-delay time only,
> > nothing
> > about altering the entire STP equation - "Configure the forward-delay
> time
> > of a Vlan X under 16 seconds." Taking the task into consideration, I
> > would
> > just alter the forward-time to a value of 7.
> >
> > I understand what you're trying to do, I'm just not sure that your
> answer
> > would meet the requirements of this task.
> >
> > Also, Aamir - to answer the 2nd part of your original post - "do we
> need
> > to
> > issue this command on both the switches or only on where the Vlan X
> > exists."
> > - while, technically, you would only need to apply it to a switch
> where
> > the
> > vlan exists - personally - as long as the vlan in question is able to
> be
> > on
> > the 2nd switch (no vlan pruning, or the switch w/out the vlan is a VTP
> > transparent switch, etc...), I would apply the timer to both switches.
> > Or, if I was really questioning it, I would ask the proctor something
> > along
> > the lines of "Should I take into consideration the chances of that
> vlan
> > being used on the 2nd switch in the future?...."
> >
> > HTH,
> > Sean
> >
> > ----- Original Message -----
> > From: "ROCHA Leandro ORANGE-FT" <leandro.rocha@orange-ft.com>
> > To: <ccielab@groupstudy.com>
> > Sent: Wednesday, August 30, 2006 10:05 AM
> > Subject: RE: Forward-delay time
> >
> >
> > > I think we have to take in account the max-age timer too.
> > >
> > > To have less than 16 seconds in any case, we should put max-age to 6
> and
> > > fwd-delay to 4 (the minimum values).
> > >
> > > Then we have a total of 6+4+4=14.
> > >
> > > Leandro
> > >
> > >
> > >
> > >
> > >
> > > "Brian McGahan" <bmcgahan@internetworkexpert.com>
> > > Sent by: nobody@groupstudy.com
> > > 08/30/06 10:39 AM
> > > Please respond to "Brian McGahan"
> > >
> > >
> > > To: "Aamir Aziz" <aamiraz77@gmail.com>,
> > <ccielab@groupstudy.com>
> > > cc:
> > > bcc:
> > > Subject: RE: Forward-delay time
> > >
> > >
> > >
> > > The forward-delay command configures each of the listening and
> > > learning phase timers. This means that if you have it configured as
> 15
> > > that it will take 30 seconds to go through both phases. If you want
> to
> > > move from blocking to forwarding in less than 16 seconds your
> forward
> > > delay would have to be 8 or lower.
> > >
> > > Brian McGahan, CCIE #8593
> > > bmcgahan@internetworkexpert.com
> > >
> > > Internetwork Expert, Inc.
> > > http://www.InternetworkExpert.com
> > > Toll Free: 877-224-8987 x 705
> > > Outside US: 775-826-4344 x 705
> > > 24/7 Support: http://forum.internetworkexpert.com
> > > Live Chat: http://www.internetworkexpert.com/chat/
> > >
> > >
> > > > -----Original Message-----
> > > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
> Behalf
> > > Of
> > > > Aamir Aziz
> > > > Sent: Wednesday, August 30, 2006 6:48 AM
> > > > To: ccielab@groupstudy.com
> > > > Subject: Forward-delay time
> > > >
> > > > Hi there,
> > > >
> > > > If the task says Configure the forward-delay time of a Vlan X
> under 16
> > > > second then what value do we use 15 or 16?
> > > >
> > > > #spanning-tree VLAN X forward-time 15 or 16
> > > >
> > > > since by default it is 15, and do we need to issue this command on
> > > both
> > > > the
> > > > switches or only on where the Vlan X exists.
> > > >
> > > > Thanks
> > > > Aamir
> > > >
> > > >
> > >
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-- 
Petr Lapukhov, CCIE #16379
petr@internetworkexpert.com

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