Re: STP Root port selection - missing step?

Daniel,
Thanks for responding. In my step 3rd step, there are two steps combined:
designated bridge's lowest port priority, then lowest port id. But if those
two are the same then it looks like the 4th step is to check the LOCAL
bridge's port priority and then the port id. So basically what I'm saying
is that there are 3 ways to manipulate the Root port selection:
Port-Priority on the designated switch, interface's cost on the local
switch and port-priority on the local switch.

Most docs i've seen only talk about the first two methods and nothing about
the 3rd.

Thanks

Tom

On Wed, Apr 4, 2012 at 11:48 PM, <daniel.dib_at_reaper.nu> wrote:

> On Wed, 4 Apr 2012 23:24:00 -0500, Tom Kacprzynski wrote:
>
>> Hello
>> I'm trying to figure this out something regarding root port selection. The
>> root port selection per most documents are:
>>
>> 1. Lowest cumulative root path cost (sum of received cost + incoming
>> port's
>> cost). This step selects the best path.
>> 2. Lowest upstream Bridge ID (designated bridge). This selects a unique
>> designated switch.
>> 3. Lowest upstream port ID (designated bridge's port id). This selects a
>> unique switch port, which should become the root port.
>>
>> My questions is what happens if you reach step 3 and everything is still
>> equal?
>>
>> How is this possible? If you have a simple topology like SW1----SW2===SW3,
>> where
>>
>> - SW1 is the root with only one connection to SW2.
>> - SW2 has spanning tree disabled.
>> - SW3 has two connection to SW2 (Fa0/20 and Fa0/21)
>>
>>
>> When I did this in a lab, SW3 would see SW1 as the designated and root
>> bridge. SW3 would always pick the lowest port, in my example fa0/20
>> instead
>> of fa0/21. So is there a step 4 to the root port selection process, where
>> it will pick the lowest port id of the local switch?
>>
>>
>> Thanks
>>
>> Tom
>>
>
> Hi Tom,
>
> Yes the 4th step is to choose the lowest port. Since the port ID could
> also be a tie there must be something certain to break the tie. I am
> surprised that you would find documentation not describing this step since
> it is essential to STP. There are some good documents on STP @ Ciscos
> website and I would also recommend reading anything Petr Lapukhov has
> written on STP.
>
> So if you want to manipulate path to root you can either change cost,
> change upstream port priority. In the lab they might tell you to "You can't
> configure switch x" which would point you to the method they are asking for.
>
> Good luck.
>
> Best regards,
>
> Daniel
>
>
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Received on Thu Apr 05 2012 - 00:53:42 ART