From: Nate Cielieska (ncielieska@gmail.com)
Date: Thu Jul 17 2008 - 02:21:47 ART
Petr,
That bridges some gaps sir.. thanks a bunch..
sorry for the misinformation Chris
Regards,
Nate
On Thu, Jul 17, 2008 at 1:07 AM, Petr Lapukhov <petr@internetworkexpert.com>
wrote:
> Nate,
> We need to clearly separate direct and indirect link failures detection
> procedures with classic STP.
>
> If a switch has redundant paths to the root (i.e. has blocking ports), then
> as soon as it detects a *root* port failure *directly* (e.g. interface goes
> down) it moves the blocked ports into listening state. The port that
> receives better root bridge information will eventually be moved to learning
> and forwarding state. If there are no alternative paths to the root, the
> switch will declare itself a new root and start sending BPDUs accordingly
> out of all its ports. Therefore, to recover from a *direct* root link
> failure it would take 2xForwadingTime seconds. Note that if a non-blocking
> designated port fails, this will just generate topology change notification
> and may affect a downstream switch (provided that it can detect this failure
> directrly).
>
> As for indirect failures. Classically, the detection of indirect failure is
> based on counting missing BPDUs on the root port. The maximum time a port
> stores BPDU information is MaxAge-BPDUAge (the age reported in BPDUs
> received on the root port). Therefore it takes MaxAge-BPDUAge seconds to
> detect that we lost connectivity to the root bridge + 2xForwardingTime to
> move best alternative blocking port into forwarding state. Assuming the
> worst case it's around MaxAge + 2xForwardingTime delay (50 secodns by
> default).
>
> However, as you remember, BackboneFast feature was specifically designed to
> speed up indirect link failures detection. As soon as a switch receives an
> inferior BPDU on one of it's designated ports (signalig that some other
> switch in the network lost connectivity to the root bridge) the switch may
> start RLQ procedure looking for a new root port and effectively defeat the
> MaxAge penalty. Therefore, with BackboneFast feature, indirect link failure
> detection may take approximately the same 2xForwardingTime seconds (maybe a
> bit longer).
>
> As for the question above, I believe it was with respect to direct link
> failures only. This is why it is enough just to change the ForwardingTime
> timer on the root switch in the topology. However, if you are extra
> cautinous, you may enable BackboneFast feature on all switches in the
> topology. Naturally, the last action only has sense if you have 3 or more
> switches.
>
> --
> Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice)
> petr@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
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>
> 2008/7/17 Nate Cielieska <ncielieska@gmail.com>:
>
> Chris,
>>
>> Based on the fact that a failure constituted the STP topology change.. and
>> we are talking about Classical STP (as opposed to RSTP) i would say that
>> your maxage comes into play here. Based on failure the way i understand it
>> is that STP needs to do a sanity check to make sure that the port going
>> down
>> is validly going down and adjust its topology accordingly, afterwhich your
>> forward delay kicks in.
>>
>> Contrast to that if you plugged a new port into a switch, there is nothing
>> to age out.. so forward-delay x2 and then forwarding would take
>> precedence.
>>
>> I would be interested to hear on the theory of other master blasters but
>> the
>> quick way i think about it is:
>>
>> Existing port into a failure condition -> Maxage applies
>> New port coming into the envionrment -> No Maxage countdown.
>>
>> Regards,
>> Nate
>>
>>
>>
>> On Wed, Jul 16, 2008 at 5:26 PM, cciestudy <cciestudy@mid-world.net>
>> wrote:
>>
>> > I would read the question to cover only how long it takes for a specific
>> > port to go from blocking to forwarding, which would be the listening and
>> > learning timers. (2x4sec = 8sec). So, I would set the forward STP timer
>> on
>> > the root bridge to 4 sec.
>> >
>> > -----Original Message-----
>> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>> > Christopher Copley
>> > Sent: Wednesday, July 16, 2008 2:34 PM
>> > To: ccielab@groupstudy.com
>> > Subject: Basic STP question
>> >
>> > All,
>> >
>> > This is one of those basic questions that you dont think about until you
>> > have to do it but I am unclear of a requirement and would like everyones
>> > thoughts. STP convergence, I have a question that reads as follows...
>> >
>> > Configure your Layer 2 network to forward packets in 8 seconds after a
>> > failure.
>> >
>> > My thinking is I would set the Hello time to 1sec and the forward time
>> to 4
>> > seconds and leave max age alone. Is this correct? and also should I
>> modify
>> > the hello time at all? I set this up in my home lab and it appears to
>> > work, but does it meet the requirements of the question?
>> >
>> >
>> > Thanks,
>> > Chris
>> >
>> >
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