GroupStudy.com GroupStudy.com - A virtual community of network engineers
 Home  BookStore  StudyNotes  Links  Archives  StudyRooms  HelpWanted  Discounts  Login
Re: MST forwarding times CCO example confusing posted 11/03/2008
[Chronological Index] [Thread Index] [Top] [Date Prev][Date Next] [Thread Prev][Thread Next]


Hi Petr,
 
Many thanks for your explaination. It makes much better sense now.
 
Regards,


--- On Mon, 11/3/08, Petr Lapukhov <petr@xxxxxxxxxxxxxxxxxxxxxx> wrote:

From: Petr Lapukhov <petr@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: MST forwarding times CCO example confusing
To: "Huan Pham" <Huan.Pham@xxxxxxxxxxxxxxxxxxxx>
Cc: "Cisco certification" <ccielab@xxxxxxxxxxxxxx>
Date: Monday, November 3, 2008, 8:38 PM

Huan,
SW2 designated ports (16-18) start setting the proposal bit in all outgoing
BPDUs as soon as the switch moves them into discarding state. However, only
port 16 on SW3 is non-blocking, and thus may respond to the proposal. Other
ports (17,18) are in discarding state (alternate role) and thus cannot
respond to the RSTP proposals. If you execute "debug spann mstp
proposal" on
both switches, you will see that SW2 actually sends proposals out of all
designated ports when you shut port 13, but SW3 only replies back on the
non-blocking uplink. This forces SW2 to bring the remaining designated ports
through the slow cycle of listening and learning (yet they will continue
sending proposals all the time!)

HTH

--
Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice)
petr@xxxxxxxxxxxxxxxxxxxxxx

Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Outside US: 775-826-4344

2008/11/3 Huan Pham <Huan.Pham@xxxxxxxxxxxxxxxxxxxx>

> Hi GS,
>
> Thanks Petr very much for your explaination. I also have read your post
> at the IE blog on PVST, and MST as well. There's lots of info I did
not
> know before. I tried your second scenario, it seems that what I observes
> is a bit different from your 2nd conclusion.
>
> What I see is that by default (point-to-point link type), only the
> designated port which corresponds to the root port of downstream switch
> (Fa0/16 in my case) goes directly from Blocking to Forwarding. All other
> designated ports (fa0/17-18) still go through
> listening/learning/forwarding states.
>
> When I changed the link type to "shared", then all designated
ports
> (fa0/16-18) go though listening/learning/forwarding. Is this normal
> behaviour?
>
> Thanks again all for your great help!
>
>
> My observation is as follow:
>
>
>
>  ROOT
>  ____    fa0/13-15     ____   fa0/16-18        ____
> |    |----------------|    |------------------|    |
> |SW1 |----------------|SW2 |------------------|SW3 |
> |____|----------------|____|------------------|____|
>
>
> (By setting the forwarding timer to 9 sec, I can turn the ports from
> blocking to forwarding in 18 sec.)
>
> SW2#sh spanning-tree
>
> MST0
>  Spanning tree enabled protocol mstp
>  Root ID    Priority    24576
>             Address     0014.6aac.1280
>             Cost        0
>             Port        15 (FastEthernet0/13)
>             Hello Time   2 sec  Max Age 20 sec
>                      Forward Delay  9 sec
<<<<<<<<<<<<
>
>  Bridge ID  Priority    32768  (priority 32768 sys-id-ext 0)
>             Address     0014.a86b.df00
>             Hello Time   2 sec  Max Age 20 sec  Forward Delay  9 sec
>
> Interface        Role Sts Cost      Prio.Nbr Type
> ---------------- ---- --- --------- --------
> --------------------------------
> Fa0/13           Root FWD 200000    128.15   P2p
> Fa0/14           Altn BLK 200000    128.16   P2p
> Fa0/15           Altn BLK 200000    128.17   P2p
> Fa0/16           Desg FWD 200000    128.18   P2p
> Fa0/17           Desg FWD 200000    128.19   P2p
> Fa0/18           Desg FWD 200000    128.20   P2p
>
>
> SW1(config)#int fa0/13
> SW1(config-if)#shut
>
>
> SW2#
> Nov  3 03:06:51: MST[0]: Fa0/13 state change forwarding -> disabled
> Nov  3 03:06:51: MST[0]: Fa0/13 state change disabled -> blocking
> Nov  3 03:06:51: MST[0]: Fa0/16 state change forwarding -> blocking
> Nov  3 03:06:51: MST[0]: Fa0/17 state change forwarding -> blocking
> Nov  3 03:06:51: MST[0]: Fa0/18 state change forwarding -> blocking
> Nov  3 03:06:51: MST[0]: Fa0/14 state change blocking -> forwarding
> Nov  3 03:06:51: MST[0]: Fa0/16 state change blocking -> forwarding
> Nov  3 03:07:00: MST[0]: Fa0/17 state change blocking -> learning
> Nov  3 03:07:00: MST[0]: Fa0/18 state change blocking -> learning
> Nov  3 03:07:09: MST[0]: Fa0/17 state change learning -> forwarding
> Nov  3 03:07:09: MST[0]: Fa0/18 state change learning -> forwarding
>
>
>
>
> SW2(config)#int range fa0/16-18
> SW2(config-if-range)#span link-type share
>
> SW1(config)#int fa0/13
> SW1(config-if)#shut
>
> SW2#
> Nov  3 03:09:05: MST[0]: Fa0/13 state change forwarding -> disabled
> Nov  3 03:09:05: MST[0]: Fa0/13 state change disabled -> blocking
> Nov  3 03:09:05: MST[0]: Fa0/16 state change forwarding -> blocking
> Nov  3 03:09:05: MST[0]: Fa0/17 state change forwarding -> blocking
> Nov  3 03:09:05: MST[0]: Fa0/18 state change forwarding -> blocking
> Nov  3 03:09:05: MST[0]: Fa0/14 state change blocking -> forwarding
> Nov  3 03:09:14: MST[0]: Fa0/16 state change blocking -> learning
> Nov  3 03:09:14: MST[0]: Fa0/17 state change blocking -> learning
> Nov  3 03:09:14: MST[0]: Fa0/18 state change blocking -> learning
> Nov  3 03:09:23: MST[0]: Fa0/16 state change learning -> forwarding
> Nov  3 03:09:23: MST[0]: Fa0/17 state change learning -> forwarding
> Nov  3 03:09:23: MST[0]: Fa0/18 state change learning -> forwarding
>
>
>
>
>
>
>
> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
> Petr Lapukhov
> Sent: Sunday, 2 November 2008 10:54 PM
> To: Huan Pham; Cisco certification
> Subject: Re: MST forwarding times CCO example confusing
>
> 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@xxxxxxxxxxxxxxxxxxxxxx
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987
> Outside US: 775-826-4344
>
> 2008/11/2 Huan Pham <pnhuan@xxxxxxxxx>
>
> > 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@xxxxxxxxx> wrote:
> >
> > From: Cyrus <cyrus.mgh@xxxxxxxxx>
> > Subject: Re: MST forwarding times CCO example confusing
> > To: "Huan Pham" <pnhuan@xxxxxxxxx>
> > Cc: "stephen skinner" <stephenski@xxxxxxxxx>,
"shiran guez"
> > <shiranp3@xxxxxxxxx>, "Cisco certification"
<ccielab@xxxxxxxxxxxxxx>
> > 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@xxxxxxxxx>
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@xxxxxxxxx> wrote:
> >
> > From: shiran guez <shiranp3@xxxxxxxxx>
> > Subject: Re: MST forwarding times CCO example confusing
> > To: "stephen skinner" <stephenski@xxxxxxxxx>
> > Cc: "Cisco certification" <ccielab@xxxxxxxxxxxxxx>
> > 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_paper09
> > 186a008
> >
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@xxxxxxxxx>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
>
>
> 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


Blogs and organic groups at http://www.ccie.net

_______________________________________________________________________
Subscription information may be found at:
http://www.groupstudy.com/list/CCIELab.html