Re: STP Cost

OK, sorry, i left out something. I am trying to get SW2 to choose the path
through SW1 to SW3 as opposed to through SW4 without making any changes on
SW2.

 Let's say SW4 has the lower BID and SW2 chooses the path through it to SW3
when all the costs are left as defaults.

I get this part "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."

To build on this; if i go in and change the cost on the SW1/SW3 Link to 5 on
SW1 then i can see the path change going through SW1 on SW2. However, if i
set everything back to default, and change only the cost of the SW1/SW2 link
on SW1 to 5, the SW2 switch still uses the path through SW4 to get to SW3.

I am getting stuck on why does it matter if i apply the cost of 5 on SW1 to
the link between SW1/SW3 or on the SW1/SW2 link? Shouldnt it just care
about the aggerate cost?

Thanks,
Don

On Fri, Oct 16, 2009 at 5:10 PM, Joe Astorino <jastorino_at_ipexpert.com>wrote:

> 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.
> Cell: +1.586.212.6107
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Received on Fri Oct 16 2009 - 17:24:57 ART