Re: STP Cost

Awesome. That was the missing link, i didnt grasp the concept of inbound vs
outbound on the STP link costs. Good tip about priorities too.

I am going to test this out again.

Thanks Joe!
Don.

On Fri, Oct 16, 2009 at 5:46 PM, Joe Astorino <jastorino_at_ipexpert.com>wrote:

> 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.
>>>>> >>
>>>>> >>
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>>>>> >>
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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

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Received on Fri Oct 16 2009 - 23:56:05 ART