Re: STP Cost

Hey Donald!

Can you break down a few more things for us? When you say the SW2/SW3 link
fails and on SW2 the root port becomes the link to SW1 do you mean for VLANs
10,20,30,40 or for VLANs 50,60 ??? What are the STP port costs of all the
interfaces in this diagram? Remember there are 2 sides to every link, and
each side could have a different cost so saying the SW1/SW2 link for
instance doesn't tell us the whole story.

Let's say for instance that these are all FastEthernet links with a default
cost of 19. Take the path SW2 -- SW1 -- SW3 as an example. SW3 as a root
bridge for vlans 10-40 advertises a cost of 0 to SW1. SW1 gets the BPDU
with a cost of 0 and adds on it's port cost of 19. SW1 sends the
information to SW2 with a cost of 19. SW2 then adds it's interface cost of
19 for a total cost of 38. Therefore, changing the port cost on SW1/SW3
link influences the total path cost.

On Fri, Oct 16, 2009 at 4:15 PM, Donald Virgil <d.virgil88_at_gmail.com> 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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