From: CCIE KH49279 (ccie_lab@inetiq.com)
Date: Tue Apr 04 2006 - 17:33:15 GMT-3
Brian et al.,
Most of this is coming together nicely, thanks for the help. I did a very
poor job of explaining myself, so I appreciate your efforts in sifting
through my verbiage and still supporting me.
Regards,
Wayne
-----Original Message-----
From: Brian McGahan [mailto:bmcgahan@internetworkexpert.com]
Sent: Tuesday, April 04, 2006 1:26 PM
To: CCIE KH49279; ccielab@groupstudy.com
Subject: RE: STP and Preferences
> I will assume that I have three ways to make a particular port
prefered
> over
> another.
> 1) interface command spanning-tree port-priority n to influence the
> natural
> selection
> 2) interface command spanning-tree cost n to influence the natural
> selection
> 3) allow the tie-breaker to occur being lowest mac address for the
> interface.
When selecting a root port a bridge uses a three step decision
process:
1. Lowest cost to the root
2. Lowest upstream bridge ID
3. Lowest upstream port ID
Cost to the root is calculated similarly to how OSPF cost works.
You take the cost that the upstream device is sending to you, add your
local interface cost, and you get the total root path cost. The
interface with the lowest root path cost wins. If you modify the cost
on your local interface it will affect your local path selection.
If there is a tie then you look at the bridge IDs of the
upstream bridges, the lowest being the winner. If there is a tie at
this point it means that you have multiple ports connected to the same
upstream bridge, as the BID must be unique to the entire STP domain.
Lastly you look at the port ID, which is made up of a port
number and a port priority. The lower priority is chosen first, and if
there is a tie the lowest port number is chosen. The key here is that
it is the *upstream* port id, not the local one. This means that if you
modify the port priority on an upstream facing interface it will not
have any effect. If you modify it on a downstream facing interface (a
designated port) it will affect the downstream neighbor's root port
election.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> CCIE KH49279
> Sent: Monday, April 03, 2006 10:58 PM
> To: ccielab@groupstudy.com
> Subject: STP and Preferences
>
> Group,
>
> I just want to make sure I am understanding something. I am sure I do,
but
> the warm fuzzy is necessary right now.
>
> For STP on a 3550.
>
>
> Root Bridge
>
> If I have a two trunk links between sw1 and sw2 and I want a
particular
> bridge (in this case sw1) to be root
> 1) I could use the 'spanning-tree vlan x,y,z root primary command to
make
> the bridge root bridge for the vlans x,y,z.
> 2)I can also use the 'spanning-tree vlan x,y,z priority n' global
command
> to
> assign a value to influence the natural election process for the root
> bridge.
>
> Port Preference
>
> I will assume that I have three ways to make a particular port
prefered
> over
> another.
> 1) interface command spanning-tree port-priority n to influence the
> natural
> selection
> 2) interface command spanning-tree cost n to influence the natural
> selection
> 3) allow the tie-breaker to occur being lowest mac address for the
> interface.
>
>
> TIA,
>
> Wayne
>
>
> Dream as if you'll live forever. Live as if you'll die today. - James
Dean
>
>
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