From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Sun Nov 02 2008 - 09:53:33 ARST
Huan,
The situation you illustrated has to deal with RSTP's way of processing
superior BPDUs - that is, BPDU received on alternate ports and bearing
better root bridge information. RSTP immediately accepts superior BPUD
information and starts downstream synchronization by blocking all non-edge
designated ports followed by proposal/agreement process. Thus, with
RSTP/MSTP your alternate port will almost immediately take the role of the
root port upon receiving the superior information. Note that this process
does not depend on the link type - be it point-to-point or shared.
This is in contrast with classic STP, which forces alternate port through
the series of LISTENING/LEARNING/FORWARDING (the classic cycle) states once
the bridge expires old information or receives a superior BPDU. (but note
here, that STP UplinkFast allows fast transition of an alternate port to the
root port state).
As for RSTP, the two common situations where you could see the classic
"forwarding delay" timer used are the following:
1) RSTP bridge receives superior classic STP BPDU on its alternate port.
Even if this port is a point-to-point link, the switch will have to
transition it through the classic cycle.
2) RSTP bridge tries to synchronize a designated (downstream port) and this
port is non-edge and is either a SHARED link (half-duplex or explicitly
configured) or connectes to a classic STP bridge or non-RSTP capable device
(or device filtering BPDUs). In this situation, RSTP will fail perfororming
fast proposal/agreement process, and will transition the candidate
designated port through the classic cycle.
-- Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice) petr@internetworkexpert.comInternetwork Expert, Inc. http://www.InternetworkExpert.com Toll Free: 877-224-8987 Outside US: 775-826-4344
2008/11/2 Huan Pham <pnhuan@yahoo.com>
> Hi Cyrus, > > Thanks for your help. > > Assuming all switches in my scenario are running MST, did you mean that, > there > is no way we can force SW2 to wait a certain delay before turning backup > root > port into the forwarding state, in case the root port fails? > > Regards, > > > > > --- On Sun, 11/2/08, Cyrus <cyrus.mgh@gmail.com> wrote: > > From: Cyrus <cyrus.mgh@gmail.com> > Subject: Re: MST forwarding times CCO example confusing > To: "Huan Pham" <pnhuan@yahoo.com> > Cc: "stephen skinner" <stephenski@gmail.com>, "shiran guez" > <shiranp3@gmail.com>, "Cisco certification" <ccielab@groupstudy.com> > Date: Sunday, November 2, 2008, 9:32 PM > > > Well, couple of things to considerate: > > 1- if all of bridges are in RSTP mode , there is no need of timers concept > ,proposal/agreement mechanism would be used (Technically if u have p2p > links,no 802.1D bridge and properly configured edge ports in your > network).Timers concept is only used for 802.1D fallback. > > 2- MST use RSTP but "spanning-tree mst forward-time" only comes into > consideration if at least 1 STP(802.1D) switch exist in network or in case > of > an indirect links (e.g. of non p2p links : switches connected to the hub!!) > > 3-When RSTP and STP wants to interoperate ,proposal/agreement mechanism > could > not be used among them.So listening /learning timers come to play > > HTH, > > > > > On Sun, Nov 2, 2008 at 8:08 PM, Huan Pham <pnhuan@yahoo.com> wrote: > > > Hi GS, > > Thanks Shiran for the clarification in the previous post. > > Switch(config)# spanning-tree mst forward-time 18 > > Since MST uses rapid spanning tree by default, the command in discussion > only > changes the time it takes for a port from Disable to Forwarding (e.g. an > access port). Only those ports will go through that forwarding delay (if > port-fast is not enable), am I right? > > How about the backup root port, it will not need to go through this delay, > won't it? > > More specifically, let's have a simple topo, where SW1 is the root. > > SW1 ======== SW2 --------- SW3 > > Both ports on SW1 are designated. > SW2 picks one of the ports (to SW1) as the root port. The other port is > backup > root (according to RSTP). Only the root port forward traffic from SW2 to > SW1. > > When the link with the root port fails, SW2 will automatically uses the > backup > root port to forward traffic, right? > > My questions are: > > - How we influence the delay to force SW2 wait 18 sec, before turning the > backup root port to be the root port (i.e. start forwarding traffic)? > > - Is there anyway we can disable RSTP within the MST? > > Thanks for your help. > > Huan > > > --- On Sun, 10/26/08, shiran guez <shiranp3@gmail.com> wrote: > > From: shiran guez <shiranp3@gmail.com> > Subject: Re: MST forwarding times CCO example confusing > To: "stephen skinner" <stephenski@gmail.com> > Cc: "Cisco certification" <ccielab@groupstudy.com> > Date: Sunday, October 26, 2008, 6:30 PM > > The basic answer is no! > > the forward time is changing the FWD timer and that timer is used between > states, but the issue is when you use MST the default STP implementation he > use is RSTP (Rapid STP -802.1w) > > meaning that the only states you will go is from learning to forwarding > that > mean that if you change the FWD timer this will be the time it take to > overall go to FWD state. > > I suggest you will read also to get a better understanding. > > > http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a008 > 0094cfa.shtml<http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml> > > the old implementation of STP 802.1d from 1998 is not used with MST. > > hope this helped you :-) > > On Sun, Oct 26, 2008 at 8:30 AM, stephen skinner > <stephenski@gmail.com>wrote: > > > hello, > > > > can i please ask a quick question , > > > > i have been reading about the following command > > > > "spanning-tree mst forward-time" > > > > according to the CCO , it states > > > > The forwarding time specifies how long each of the listening and learning > > states last before the interface begins forwarding. > > > > OK, > > > > but the example on the CCO shows this > > > > This example shows how to set the spanning-tree forwarding time to 18 > > seconds for all MST instances: > > > > Switch(config)# spanning-tree mst forward-time 18 > > > > > > my question is , > > > > if the above statement about listening and forwarding states is correct , > > i.e this command covers the timer for both the listening and learning > > states > > > > > > then shouldnt the example have a forward time of 9 ?? > > > > > > i beleive that they way you work out the forwarding time is as follows > > > > listening=9 and learning=9 , 2x9 =18 in total. > > > > the example from the CCO would be > > > > listening=18 and learning=18 , 2x18 = 36 in total. > > > > can someone please clarify this for me ... > > > > many thanks > > > > > > -- > > Only two things are infinite, the universe and human stupidity, and > I'm not > > sure about the former. > > > > > > Blogs and organic groups at http://www.ccie.net > > > > _______________________________________________________________________ > > Subscription information may be found at: > > http://www.groupstudy.com/list/CCIELab.html > > > > > > > > > > > > > > > > > > > -- > Shiran Guez > MCSE CCNP NCE1 JNCIA-ER CCIE #20572 > http://cciep3.blogspot.com > http://www.linkedin.com/in/cciep3 > > > 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 > > > > > > > > > > > -- > Sirus Moghadasian > CCIE #21862 (R&S) > > > Blogs and organic groups at http://www.ccie.net > > _______________________________________________________________________ > Subscription information may be found at: > http://www.groupstudy.com/list/CCIELab.html
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