Re: STP Cost

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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Received on Fri Oct 16 2009 - 22:30:59 ART