Re: STP Cost

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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Received on Fri Oct 16 2009 - 17:00:14 ART