From: Bob Sinclair (bsinclair@netmasterclass.net)
Date: Fri May 28 2004 - 10:24:37 GMT-3
All,
Here is a quote from the 802.1D standard that summarizes the process:
quote:
The Bridge with the highest priority Bridge Identifier is the Root (for
convenience of calculation, this is the identifier with the lowest numerical
value). Every Bridge Port in the Bridged LAN has a Root Path Cost associated
with it. This is the sum of the Path Costs for each Bridge Port receiving
frames forwarded from the Root on the least cost path to the Bridge. The
Designated Port for each LAN is the Bridge Port for which the value of the
Root Path Cost is the lowest: if two or more Ports have the same value of
Root Path Cost, then first the Bridge Identifier of their Bridges and then
their Port Identifiers are used as tie- breakers. Thus, a single Bridge Port
is selected as the Designated Port for each LAN, the same computation
selects the Root Port of a Bridge from among the Bridge's own Ports, and the
active topology of the Bridged LAN is completely determined.
end quote
Criteria in choosing RP or DP:
1. lowest Root path cost
2. if path cost is tied, lowest sender BID
3. if sender BID equal (parallel links) look at port id.
HTH,
Bob Sinclair
CCIE #10427, CISSP, MCSE
www.netmasterclass.net
----- Original Message -----
From: "Phil" <ccie9323@yahoo.com>
To: "Tom Young" <gitsyoung@yahoo.co.jp>; <ccielab@groupstudy.com>
Sent: Friday, May 28, 2004 8:09 AM
Subject: Re: question about STP
> Tom,
>
> You have to make cat2950#1 the secondary root to
> acomplish this. First, STP checks the bridge priority
> and then the port cost as a tie breaker.
>
> Phil
>
> --- Tom Young <gitsyoung@yahoo.co.jp> wrote:
> > Hi, group
> >
> > I confused on the spantree question. I connected
> > the
> > network as below, two cat6509s and two cat2950s, the
> > cat6509#1 for the STP root and the cat6509#2 for the
> > secondary root. The port that connect cat2950#1 from
> > cat2950#2 was blocked by STP function. I want to
> > block the
> > port that from Cat6509#2 to Cat2950#2. I tried the
> > next
> > two commands, but failured, how can I do ? Help
> > me,please.
> >
> > set spantree portvlancost 3/3 cost 200
> > set spantree portcost 3/3 cost 200
> > (the port 3/3 of cat6509#2 was connected with
> > cat2950#2)
> >
> > cat6509#1------cat6509#2
> > | |
> > | |
> > cat2950#1------cat2950#2
> >
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