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 11/02/2008
[Chronological Index] [Thread Index] [Top] [Date Prev][Date Next] [Thread Prev][Thread Next]


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_paper09186a008
0094cfa.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)