From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Sun Aug 03 2003 - 18:42:33 GMT-3
Group,
Spanning tree cost and priority is a common problem area. To
understand how they work and how to apply them, we must first understand
STP's order of operations in selecting a root port. The order is as
follows:
1. Lowest cumulative cost to the root
2. Lowest bridge ID of upstream bridge
3. Lowest port priority of upstream bridge
4. Lowest local port ID
First, a bridge looks at the cumulative cost to reach the root.
This is the cost that any upstream bridge reports as its cost to the
root, plus the local cost to get to that upstream bridge. The lowest
value wins.
If there is a tie in port costs, the next value that the bridge
considers is the bridge id (mac address) of the upstream bridges. The
lowest bridge id wins.
If there is a tie in bridge id (ie you have multiple connections
to the same bridge), the next value taken into account is port priority.
This is the port priority configured on the *upstream* bridge, not the
local port priority.
If port priority is the same, the final tiebreaker is the local
port id. Lower port id wins.
Therefore, if you want to affect the path of the spanning-tree,
the following holds true:
If you change local port cost, it will affect how you are
forwarding upstream, as well as how everyone downstream of you is
forwarding. Since cost is cumulative throughout the STP domain,
changing your local cost *will* affect downstream bridges as well.
If you change local port priority, it will only affect the
downstream bridge attached to that port. If the port is connected to an
upstream bridge, changing the port priority will not have any affect.
Also, remember that the downstream bridge will only take port priority
into consideration if there is a tie for both cost and bridge id.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Cell: 708-362-1418
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
yu chunyan
Sent: Saturday, August 02, 2003 8:50 PM
To: matijevi@bellsouth.net; ccielab@groupstudy.com
Subject: Re: spanning-tree and port priority
Hi,
I donot know if you have book "cisco lan switching' written by kennedy
clark, which talk about every thing about spanning tree problem. This is
a
common mistake one ever makes about spanning tree. If you want
port-priority
to affect the root port selection, you have to set port-priority in
upstream
switch, not the switch you want to change, and this technique is only
proper
in back to back case. If you have mutiple upstream switch, it does not
work.
read the book.
Bin.
>From: "John Matijevic" <matijevi@bellsouth.net>
>Reply-To: "John Matijevic" <matijevi@bellsouth.net>
>To: "John Matijevic" <matijevi@bellsouth.net>, <ccielab@groupstudy.com>
>Subject: Re: spanning-tree and port priority
>Date: Sat, 2 Aug 2003 12:14:00 -0400
>
>Team,
>If anyone can get this to work using port-priority command please let
me
>know. Otherwise I am assuming the DocCd is wrong that the port-priority
>value modification does not make a difference.
>The only way I could change the state from forwarding to blocking was
to
>adjust the cost. In my initial thinking based on the DocCd I thought I
>could
>do this with the port-priority. I even tried using the value 112. Based
on
>the following:
>Valid priority values are 0, 16, 32, 48, 64, 80, 96, 112, 128, 144,
160,
>176, 192, 208, 224, and 240. All other values are rejected.
>
>Sincerely,
>
>Matijevic
>
>----- Original Message -----
>From: "John Matijevic" <matijevi@bellsouth.net>
>To: <ccielab@groupstudy.com>
>Sent: Saturday, August 02, 2003 11:29 AM
>Subject: spanning-tree and port priority
>
>
> > Hello Team,
> > I know there is an ealier thread on 7/7/03, on this that I read, but
no
> > response.
> > I am trying to change the state from blocking to forwarding on a
>non-root
> > switch.
> > I used the command under interface:
> > spanning-tree vlan 1 port-priority 100,
> > By lowering the priority the port should go into forwarding state
and
>the
>rest
> > should be blocking. However, this is not happening, I know there was
an
> > earlier thread on this but there was no response.
> > p1s2#sh span vlan 1
> >
> > VLAN0001
> > Spanning tree enabled protocol ieee
> > Root ID Priority 24577
> > Address 000b.be5d.5b80
> > Cost 19
> > Port 21 (FastEthernet0/21)
> > Hello Time 2 sec Max Age 20 sec Forward Delay 15
sec
> >
> > Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
> > Address 000b.46d9.2580
> > Hello Time 2 sec Max Age 20 sec Forward Delay 15
sec
> > Aging Time 300
> >
> > Interface Port ID Designated
>Port
> > ID
> > Name Prio.Nbr Cost Sts Cost Bridge ID
> > Prio.Nbr
> > ---------------- -------- --------- --- ---------
--------------------
> > --------
> > Fa0/21 128.21 19 FWD 0 24577
000b.be5d.5b80
>128.21
> > Fa0/22 128.22 19 BLK 0 24577
000b.be5d.5b80
>128.22
> > Fa0/23 128.23 19 BLK 0 24577
000b.be5d.5b80
>128.23
> > Fa0/24 100.24 19 BLK 0 24577
000b.be5d.5b80
>128.24
> >
> > p1s2#
> > port fa0/21 is the port im trying to change, on the non-root switch.
> > Any suggestions?
> > Sincerely,
> > Matijevic
> >
> >
> >
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