Re: STP Cost

So the spanning-tree cost is aggregate Donald, you are right about that.
You could very well manipulate the STP port cost JUST on SW2 to get the same
results. What results were you expecting? If there is a tie, things will
go to the lowest sent BID...in other words if the cost is say 38 from the
link SW2 -- SW1 -- SW3 and the cost is also 38 going from SW2 -- SW4 -- SW3
then we look at the lowest sending BID. So who has the lower BID SW1 or
SW4? The lower BID wins.

On Fri, Oct 16, 2009 at 5:00 PM, Donald Virgil <d.virgil88_at_gmail.com> wrote:

> Miroslav,
>
> Yes, i want the RP role from the Interface-to-SW3 to Interface-to-SW1 after
> the SW2-SW3 trunk fails.
>
> I am under the impression the cost of a path is aggerate so the cost of the
> link from SW2-SW1 and SW1-SW3 are added together and compared to other
> available paths. Is this not the case? If it is, why does it matter if i
> apply the cost on the link between SW2-SW1 or from SW1-SW3.
>
> When you say receiving side of a link, what is considered the receiving
> side
> in this case when the link between SW2 and SW3 fail?
>
> Don
>
>
>
>
> On Fri, Oct 16, 2009 at 4:30 PM, Miroslav Kosut <miroslav.kosut_at_gmail.com
> >wrote:
>
> > Hello Don,
> >
> > Let's make clear one point:
> > Do you want to get SW2 to move the RP role from the Interface-to-SW3 to
> > Interface-to-SW1 after the SW2-SW3 trunk fails ? (so assuming we are
> talking
> > about VLANs 10, 20, 30, 40 rooted at SW3)
> >
> > If so, then the the interface cost is added to the root path cost
> included
> > in the BPDU on the RECEIVING SIDE OF A LINK.
> > The cost configured on designated ports (for a particular vlan) is NOT
> USED
> > anywhere. Only the cost value configured on root ports are used when
> > calculating the root path cost.
> >
> > I hope it is more clear. Please correct me if I am wrong.
> >
> > Regards,
> > Miroslav
> >
> >
> > On Oct 16, 2009, at 10:15 PM, Donald Virgil wrote:
> >
> > I am having a hard time understanding STP cost and how links get chosen
> >> over
> >> others.
> >>
> >> Example
> >>
> >> SW1----- SW2
> >> | / |
> >> | / |
> >> | / |
> >> SW3-----SW4
> >>
> >>
> >> SW3 is root for VLAN 10, 20, 30, 40
> >> SW2 is root for VLAN 50, 60.
> >>
> >> When I set the STP cost to 5 on the trunk between SW1 and SW3 and the
> >> trunk
> >> between SW2 and SW3 fail, the STP root ports on SW2 is the trunk from
> SW2
> >> to
> >> SW1.
> >>
> >> I am confused as to why the cost has to be set on the link between SW1
> and
> >> SW3 to get this behavior instead of the link between SW1 and SW2.
> >>
> >> Thanks,
> >> Don.
> >>
> >>
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-- 
Regards,
Joe Astorino - CCIE #24347 R&S
Technical Instructor - IPexpert, Inc.
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