Re: STP Cost

To put it another way... cost only matters INBOUND not outbound. Port
priority on the other hand is the opposite -- it only matters outbound and
not inbound.

On Fri, Oct 16, 2009 at 5:45 PM, Joe Astorino <jastorino_at_ipexpert.com>wrote:

> 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
>

-- 
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