Re: STP Cost

The reason is because when you change the cost on the SW1 interface facing
towards SW2, that cost is not taken into account when SW2 calculates
things. Assume we have all the defaults, except you changed the SW1
interface facing SW2 to cost of 5. SW3 advertises out a cost of 0 to
itself, the root bridge. SW2 gets this and adds on the cost of it's SW3
facing interface, 19. SW2 then sends a cost of JUST 19 to SW1. The cost of
the outgoing link is not taken into affect here. When SW2 receives the cost
of 19, it then adds on a cost of 19 for a total of 38. Since SW4 still has
the lower BID it wins. The cost of 5 on that particular link has nothing to
do with things from the perspective of SW2

On Fri, Oct 16, 2009 at 5:24 PM, Donald Virgil <d.virgil88_at_gmail.com> wrote:

> 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.
>>> >>
>>> >>
>>> >> Blogs and organic groups at http://www.ccie.net
>>> >>
>>> >>
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>>>
>>>
>>>
>>>
>>>
>>
>>
>> --
>> Regards,
>>
>> Joe Astorino - CCIE #24347 R&S
>> Technical Instructor - IPexpert, Inc.
>> Cell: +1.586.212.6107
>> Fax: +1.810.454.0130
>> Mailto: jastorino_at_ipexpert.com
>>
>
>

-- 
Regards,
Joe Astorino - CCIE #24347 R&S
Technical Instructor - IPexpert, Inc.
Cell: +1.586.212.6107
Fax: +1.810.454.0130
Mailto:  jastorino_at_ipexpert.com
Blogs and organic groups at http://www.ccie.net