From: Shadi (ccie@xxxxxxxxxxxxxxxx)
Date: Sun Mar 17 2002 - 13:20:11 GMT-3
I know this is old, but I was searching on the same topic, why he can not
use the set spantree cost 2/1 2, that will make that 2/1 port become the
root port?
in which order does the spantree port or cost influence the root port
selection? is it the cost, if there is a tie then the priority or visa
versa?
----- Original Message -----
From: "Bob Sinclair" <bsin@erols.com>
To: <ccielab@groupstudy.com>
Sent: Friday, February 01, 2002 10:09 PM
Subject: Re: Need a good reference for Spanning Tree
> If A is the designated bridge (the root or the one closest to the root)
then on B, 1/1 will be the root port and 1/2 will block. This is controlled
by the port ids being received by switch B from Switch A. To get port 1/2
to be the root port on Switch B, lower the port priority on port 1/2 of
Switch A: set spantree portpri 1/2 20
>
> All else being equal, STP prefers ports with the lowest port ID, which is
a 16 bit number made up of port priority and an index number. Default port
priority is 32 on modular Cats. Index increases left to right across the
module.
>
> -Bob
>
>
> ----- Original Message -----
> From: "SEAN" <nyboi@excite.com>
> To: <ccielab@groupstudy.com>
> Sent: Friday, February 01, 2002 2:16 PM
> Subject: Need a good reference for Spanning Tree
>
>
> > Can anyone recommend a good reference to how spanning tree chooses which
> > ports to go up to the root bridge?
> >
> > Basically, I have the following set up - two catalyst 5500 switches with
> > two trunks between them:
> > Switch A port 1/1 to Switch B port 1/1
> > Switch A port 2/1 to Switch B port 2/1
> >
> > Seems the switch always wants to use 1/1 as the spanning tree root
> > (unless it's not available). If 1/1 is disabled, and 2/1 is forwarding
> > fine, and I reenable 1/1, spanning tree recalculates, and 1/1 becomes
> > forwarding again.
> >
> > These are supervisor III's, and I was thinking that it probably wants to
> > use the ports with the lowest mac address? But if I do show mod, both
> > supervisor modules show the same mac address range.
> >
> > Can anyone recommend a good resource on how spanning tree chooses ports?
> >
> > ------------------------------------------------------------------------
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