Spanning Tree Priority change

Spanning Tree Priority change
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Spanning Tree Priority change daytime 02-04-2007
Posted by daytime on February 4, 2007, 4:10 am
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Hello- I saw this question on a mock exam about why would an
administrator change the value of the spanning tree priority of a
switch.(it didnt specify how it would change the value)
I thought the answer would be to make/deny the switch to be the root
bridge.
Here is what the wiki says

Each switch has a MAC address and a configurable priority number; both
of these numbers make up the Bridge Identification or BID. The BID is
used to elect a root bridge based upon the lowest priority number; if
this is a tie then the numerically lowest MAC address wins. Because it
is next to impossible that two MAC addresses will be the same (they
are uniquely assigned), one switch should always be successfully
elected as the root bridge with the switch that has the next lowest
priority/cost acting as a secondary (backup) root bridge. Other
switches in the network that use spanning tree will use the same
algorithmic process to calculate the shortest path to the root bridge
and so produce a loop free tree topology where multiple paths to the
root bridge exist. The priority number is normally left at its default
value but can be reconfigured to a lower number if the network
administrator wishes a particular switch to be elected; otherwise the
whole process is fully automated.

However the answer was -in order to optimize the path that frames take
from source to destination

I am wrong to think that you would only change the priority to win/
lose the election process


Posted by Den on February 4, 2007, 4:38 am
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> Hello- I saw this question on a mock exam about why would an
> administrator change the value of the spanning tree priority of a
> switch.(it didnt specify how it would change the value)
> I thought the answer would be to make/deny the switch to be the root
> bridge.
> Here is what the wiki says
>
> Each switch has a MAC address and a configurable priority number; both
> of these numbers make up the Bridge Identification or BID. The BID is
> used to elect a root bridge based upon the lowest priority number; if
> this is a tie then the numerically lowest MAC address wins. Because it
> is next to impossible that two MAC addresses will be the same (they
> are uniquely assigned), one switch should always be successfully
> elected as the root bridge with the switch that has the next lowest
> priority/cost acting as a secondary (backup) root bridge. Other
> switches in the network that use spanning tree will use the same
algorithmic process to calculate the shortest path to the root bridge
> and so produce a loop free tree topology where multiple paths to the
> root bridge exist. The priority number is normally left at its default
> value but can be reconfigured to a lower number if the network
> administrator wishes a particular switch to be elected; otherwise the
> whole process is fully automated.
>
> However the answer was -in order to optimize the path that frames take
> from source to destination
>
> I am wrong to think that you would only change the priority to win/
> lose the election process

In a broad sense, isn't that correct? it says the process is automated
unless the administrator changes it.
So if you change it, you are giving higher priority to that switch to become
the root bridge, hence you are optimsing the path the frames take. (or am I
missunderstanding what you are saying?)
This is the command to change the BID:
router(config)#spanning-tree vlan 1 priority 4096



Posted by BernieM on February 4, 2007, 5:25 am
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> Hello- I saw this question on a mock exam about why would an
> administrator change the value of the spanning tree priority of a
> switch.(it didnt specify how it would change the value)
> I thought the answer would be to make/deny the switch to be the root
> bridge.
> Here is what the wiki says
>
> Each switch has a MAC address and a configurable priority number; both
> of these numbers make up the Bridge Identification or BID. The BID is
> used to elect a root bridge based upon the lowest priority number; if
> this is a tie then the numerically lowest MAC address wins. Because it
> is next to impossible that two MAC addresses will be the same (they
> are uniquely assigned), one switch should always be successfully
> elected as the root bridge with the switch that has the next lowest
> priority/cost acting as a secondary (backup) root bridge. Other
> switches in the network that use spanning tree will use the same
> algorithmic process to calculate the shortest path to the root bridge
> and so produce a loop free tree topology where multiple paths to the
> root bridge exist. The priority number is normally left at its default
> value but can be reconfigured to a lower number if the network
> administrator wishes a particular switch to be elected; otherwise the
> whole process is fully automated.
>
> However the answer was -in order to optimize the path that frames take
> from source to destination
>
> I am wrong to think that you would only change the priority to win/
> lose the election process
>

the answer is getting to the end result ... you change the priority to
ensure a particular switch wins the election process ... influencing the
layer-2 topology and therefore optimizing the path.



Posted by daytime on February 4, 2007, 10:18 am
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>
>
>
>
> > Hello- I saw this question on a mock exam about why would an
> > administrator change the value of the spanning tree priority of a
> > switch.(it didnt specify how it would change the value)
> > I thought the answer would be to make/deny the switch to be the root
> > bridge.
> > Here is what the wiki says
>
> > Each switch has a MAC address and a configurable priority number; both
> > of these numbers make up the Bridge Identification or BID. The BID is
> > used to elect a root bridge based upon the lowest priority number; if
> > this is a tie then the numerically lowest MAC address wins. Because it
> > is next to impossible that two MAC addresses will be the same (they
> > are uniquely assigned), one switch should always be successfully
> > elected as the root bridge with the switch that has the next lowest
> > priority/cost acting as a secondary (backup) root bridge. Other
> > switches in the network that use spanning tree will use the same
> > algorithmic process to calculate the shortest path to the root bridge
> > and so produce a loop free tree topology where multiple paths to the
> > root bridge exist. The priority number is normally left at its default
> > value but can be reconfigured to a lower number if the network
> > administrator wishes a particular switch to be elected; otherwise the
> > whole process is fully automated.
>
> > However the answer was -in order to optimize the path that frames take
> > from source to destination
>
> > I am wrong to think that you would only change the priority to win/
> > lose the election process
>
> the answer is getting to the end result ... you change the priority to
> ensure a particular switch wins the election process ... influencing the
> layer-2 topology and therefore optimizing the path.

Thanks alot for that-I was just thinking that changing the priority
would make the switch the root ,not looking at the bigger picture.
thanks for your feedback-much appreciated.


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other useful resources:
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