From: dave dave (funccie@gmail.com)
Date: Wed Feb 13 2008 - 02:12:24 ARST
Hi,
DS1 ---- DS2
! /
! /
SW1
DS1 -- Root Bridge all port will be DP
DS2 : - ds2 to ds1 port will be root port closer to the RB in term of low
cummulative root path cost (Only one RP each Non RB switch )
SW1 : - sw1 to ds1 port will be root port closer to the RB in term of low
cummulative root path cost (Only one RP each Non RB switch )
Now you have the sw1 to ds2 & ds2 to sw1 remain for STP (our manual ;)
) calculation
SW1 To (RB) DS1 cost = 3 Therefore the port on segment sw1 to ds2
will be DP
DS2 To (RB) DS1 cost = 4 Therefor the port on segment ds2 to sw1
will be NDP (blocking port)
If you want desited result, mean blocking sw1 to ds2 port & unblock ds2 to
sw1 port,
1. Lower cost below 3 between ds1 & ds2
or
2. Higher the cost above 4 between ds1 to sw1
HTH
Dave
On 2/8/08, thomas.rader@freesurf.ch <thomas.rader@freesurf.ch> wrote:
>
> hi,
>
> I thought I understood the STP cost concept until I labed a simple
> scenario and couldn't work out why ports were being blocked.
>
> Here is what I did:
>
> My scenario has an access switch (SW1) connected to two distribution
> switches (DS1 & DS2) via 2xgig etherchannel (stp cost 3)
> The distributibution switches (DS1 & DS2) are also connected via gig trunk
> (stp cost 4).
> DS1 is STP root.
> I used the "spanning-tree cost" under ther interface or in the case of
> etherchannel on the port-channel.
>
>
> If I change the STP cost on SW1 etherchannel connected to DS2 to 1000, I
> would expect that STP blocks SW1 port to DS2.
> However in fact it blocks DS2 connected to SW1. Why ?
>
> Here is how I see it:
> SW1 to DS1 = cost 3
> SW1 to DS1 (via DS2) = cost 1004 (1000 + 4 crossing the DS2-DS1 link)
> DS2 to DS1 = cost 4
> DS2 to DS1 (via SW1) = cost 6 (3 + 3)
>
> This assumes that only the outgoing cost per hop is added to the cost. Is
> this correct ?
>
> I have seen some confilicting information regarding per VLAN
> ("spanning-tree vlan 1-1024 cost"). The way I see it you can simply control
> this better per per VLAN. Right ?
>
> Also why does STP only block one side of the connection ? I would have
> expected both sides of a connection to be blocked ?
>
>
> thanks, thomas
>
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