You won't see any problems with max age if you leave it at the default of
20 seconds.
The seven hops comes from conservative estimates of how long it takes a
bridge to take in the BPDU and propagate it. With 7 hops each could take
about 3 seconds. There's no way that switches these days take that long. In
fact, switches don't even bother to figure out how long it takes. They just
add 1 second.
I am currently dissecting STP! ;-)
Priscilla
At 07:32 PM 8/29/01, Leigh Anne Chisholm wrote:
> From other statements I've read (Cisco published material) and from the
>original excerpt I published, I'd imagine that the placement of the root
>does matter.
>
>"Part of this restriction is coming from the age field BPDU carry:
>when a BPDU is propagated from the root bridge towards the leaves of the
>tree, the age field is incremented each time it goes though a bridge.
>Eventually, when the age field of a BPDU goes beyond max age, it is
>discarded. Typically, this will occur if the root is too far away from some
>bridges of the network. This issue will impact convergence of the spanning
>tree."
>
>I'd think that if a bridge were to be the third bridge away from the root,
>and another switch was the third bridge on the far side of the root, I
>wouldn't expect to see any problems with MaxAge because I can't see the root
>being too far from some of the bridges in the network. Now if a bridge were
>to be the seventh, I could see how that would impose a greater delay and
>possibly negatively impact the MaxAge parameter. Now my question would
>be... does this really apply in today's networks or is this more of a
>limitation of yesteryear's "software-based bridges"?
>
>One day I'll dissect STP inside and out. One day...
>
>
> -- Leigh Anne
>
> > -----Original Message-----
> > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> > Priscilla Oppenheimer
> > Sent: Wednesday, August 29, 2001 3:17 PM
> > To: [EMAIL PROTECTED]
> > Subject: Re: What's the diameter of your switched network? [7:17489]
> >
> >
> > At 07:27 PM 8/28/01, Gareth Hinton wrote:
> > >Always thought that Diameter was a misleading term.
> > >If the root bridge is physically in the centre of the bridged
> > network, the
> > >diameter is actually the radius.
> > >Hmmm - more coffee - it's late.
> >
> > Hmm, it is late, but I don't think the placement of the root
> > bridge matters
> > in this question. The Cisco text says "This means that two
> > distinct bridges
> > in the network should not be more than seven hops away the one to the
> > other." The English is awkward, but the meaning is clear and notice that
> > there's no mention of the root bridge.
> >
> > IEEE 802.1D says that the recommended value for the maximum
> > bridge diameter
> > is 7, and its definition is "The maximum bridge diameter of the
> > Bridge LAN:
> > The maximum number of Bridges between any two points of attachment of end
> > stations."
> >
> > Priscilla
> >
> >
> >
> >
> > >""Leigh Anne Chisholm"" wrote in message
> > >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > > > Here's something funky I've just started researching. Thought many
of
> > you
> > > > might not be aware of this...
> > > >
> > > > Awkward STP Parameter Tuning and Diameter Issues
> > > >
> > > > We already saw that an aggressive value for the max-age
> > parameter and the
> > > > forward-delay could lead to a very unstable STP. The loss of
> > some BPDUs
> > >can
> > > > then cause a loop to appear. Another issue, not very known,
> > is related to
> > > > the diameter of the bridged network. The conservative default
> > values for
> > >the
> > > > STP impose a maximum network diameter of seven. This means that two
> > >distinct
> > > > bridges in the network should not be more than seven hops
> > away the one to
> > > > the other. Part of this restriction is coming from the age field BPDU
> > >carry:
> > > > when a BPDU is propagated from the root bridge towards the
> > leaves of the
> > > > tree, the age field is incremented each time it goes though a bridge.
> > > > Eventually, when the age field of a BPDU goes beyond max age, it is
> > > > discarded. Typically, this will occur if the root is too far away
from
> > >some
> > > > bridges of the network. This issue will impact convergence of the
> > spanning
> > > > tree.
> > > >
> > > >
> > > > This came from: http://www.cisco.com/warp/public/473/16.html#2f
> > ________________________
> >
> > Priscilla Oppenheimer
> > http://www.priscilla.com
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=17897&t=17489
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