From: Hussam EL Kebbi (hussamkibbi@hotmail.com)
Date: Thu Mar 05 2009 - 10:45:05 ARST
Hi Petr,
Thank you very much for this long clear explanation. These small things matter
in understanding
I'll be waiting your blog post this or next week, explaining the classic STP
and RSTP convergence scenarios,
it would be very helpful. By the way are there any good VoD for that issue?
> Date: Thu, 5 Mar 2009 04:52:40 -0600
> Subject: Re: STP misconcept!
> From: petr@internetworkexpert.com
> To: hussamkibbi@hotmail.com; ccielab@groupstudy.com
>
> Hi,
>
> You should remember that classic STP peforms iterative computations,
> trying to find
> a single spanning-tree root. The computations are performed by constantly
> relaying
> the "best" bridge information to all downstream neighbors. This is very
> similar to
> gradient-based optimization, where every switch accepts the best
> information at
> given moment.
>
> In order to stabilize the computations, some "inertial" mechanism is built
> into the STP
> algorithm. Specifically, every bridge is supposed to keep the last optimal
> root bridge
> information for maximum of "max_age" seconds, if it does not receive any
> superior
> (better) information. [actually, it is less than max_age, since every BPDU
> carries
> the age field, but this is just small detail, not pertaining to the
> general idea].
>
> Now imagine what happens if a link directly connected to the switch fails.
> If the port,
> where the link connects to is the root port, we have two different
situaitons
>
> 1) The switch detects the loss of carries, and thus immediately
> transitions the port
> into the down state. The root bridge information associated with this port
> is immediately
> discarded, as we know that the port is dead. If the switch has any blocked
> ports, it
> finds the one with the next "best" information stored, and transitions it
> through the
> listening+learning states, resulting in the new root port.
>
> 2) The switch does not detect the loss of carries, but loses BPDUs on the
> root port.
> In this situation, the switch cannot immediately expire the best root
> bridge information
> on the current root port, and has to wait for maximum of "max_age"
> seconds, before
> expiring the information and unblocking the alternate port.
>
> If the failed port is not the root port, the situation is usually simpler.
> If the port was blocked,
> but has lost the flow of BPDUs, the switch will eventually expire to root
> bridge information
> stored for this port, and unblocks it, making the designated port.
>
> If the failed port is the designated port, nothing much would happen. If
> the switch detect
> the loss of carrier, it will simply generate a topoly change event.
>
> Now for the MAC address table aging. It has nothing to do with the STP
> max_age concept.
> However, it is connected with some STP events. Specifically, if a switch
> detects that any
> of its ports goes up or down, it notifies all switches in the topology of
> the "topology change"
> event, by flooding the information via the root bridge. The details of the
> process are
> uniteresting, but the effect is that every switch that hears the TCN BPDU
> relayed from the root
> will cut the MAC address table aging time by "forward_delay". This will
> allow to re-learn the
> new MAC address locations, for the stations that have moved as a result of
> the topology change.
> This process results in two significant issues:
>
> 1) Additional 15 seconds might be needed to age the old information
> 2) Excessive flooding may occur due to the information being aged, and
> switches turning into
> hubs temporarily.
>
> The second issue might be very serious in the network with a lot of
> topology changes, as it might
> cause all sorts of weird behavior and intermittent network meltdowns.
>
> I did not cover the BackboneFast feature in this message, as it might take
> another page of explanations :)
> I'll try to make a blog post this or next week, explaining the classic STP
> and RSTP convergence scenarios,
> as this seems to be vaguely covered anywhere.
>
> You may want to read the following on the classic STP meanwhile:
>
>
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800
94797.shtml
>
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800
c2548.shtml#recovers
>
>
> HTH,
>
> Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice)
> petr@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987
> Outside US: 775-826-4344
>
> ----- Original Message -----
> From: "Hotmail" <hussamkibbi@hotmail.com>
> Sent: Thu, March 5, 2009 7:09
> Subject:STP misconcept!
>
>
> Hi Experts,
>
> After reading Cisco docs and passing through videos of internetexpert for
> STP, I still have some misleading concept about some parts.
>
> - Main one is the failure process: Direct - Indirect failures and how they
> relate to max-age table and MAC address age table? many scenario's makes me
> lost!
> - Enhancements in Cisco: Backbone fast (which relate to indirect failure)
> and uplink fast: I think I understand it but then I get lost!
>
> Hope you can guide me through a clear way of understanding the failure
> process as I passed through the docs/video several times and still not
> grasping it very well.
>
> Thanks,
>
> Hussam
>
>
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> ----- End of original message -----
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