From: Darby Weaver (darbyweaver@yahoo.com)
Date: Sat Mar 03 2007 - 05:02:04 ART
Here's a good description of it from the Cisco Switch
Field Manual:
7. (Optional) Tune the root path cost.
a. (Optional) Set the port cost scale (Catalyst 4000
and 6000 only):
COS set spantree defaultcostmode {short | long}
IOS (global) spanning-tree pathcost defaultcost-method
{long | short}
By default, PVST+ switches use the short (16-bit) port
cost values. If you have any ports that are 10 gbps or
greater, you should set the port cost scale to long
(32-bit) values on every switch in your network.
MISTP, MISTP-PVST+, and MST modes use long mode by
default.
b. Set the port cost for all VLANs or instances:
This should help explain the matter a bit better.
The trick seems to be that you need to ensure that all
devices in the topology agree to use long or short
port cost values.
The searches I did yielded a bit more information in
that this feature is quite common among other vendors
as well.
So...
Even though it is not well documented in the
3550/3560, it might be nice to know it as a lab topic,
I suppose.
Imagine: You change from PVST to MSTP and the default
was short (PVST) and now it is long.
Or if you configured this out of order perhaps the
value you entered for cost was initially a 32-bit
vlaue and now it is only a 16-bit value.
I guess it could cause some stirs in one's ultimate
objectives. And maybe some things not quite happening
the way one might have envisioned them.
Hmmm... after all of this, I'll likely remember this
topic well now.
Thanks
Darby
--- Darby Weaver <darbyweaver@yahoo.com> wrote:
> Here's a quick link to it on the 4000.
>
> Let me dig a bit further.
>
> I suspect it is in relation to PVST to MSTP
> Migrations
> that you are most interested in.
>
>
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat4000/12_18a/command/s1.htm#1029022
> --- nhatphuc <nhatphuc@gmail.com> wrote:
>
> > Darby,
> >
> > Thanks for replying, but I didn't mention
> > spanning-tree cost, the exact
> > command I asked is: spanning-tree pathcost method
> >
> > Thanks
> >
> > On 3/3/07, Darby Weaver <darbyweaver@yahoo.com>
> > wrote:
> > >
> > > Here you go - syntax can be important.
> > > ======================================
> > >
> > > spanning-tree cost
> > > Use the spanning-tree cost interface
> configuration
> > > command to set the path cost for spanning-tree
> > > calculations. If a loop occurs, spanning tree
> > > considers the path cost when selecting an
> > interface to
> > > place in the forwarding state. Use the no form
> of
> > this
> > > command to return to the default setting.
> > >
> > > spanning-tree [vlan vlan-id] cost cost
> > >
> > > no spanning-tree [vlan vlan-id] cost
> > >
> > > Syntax Description
> > > vlan vlan-id
> > > (Optional) VLAN range associated with a
> > spanning-tree
> > > instance. You can specify a single VLAN
> identified
> > by
> > > VLAN ID number, a range of VLANs separated by a
> > > hyphen, or a series of VLANs separated by a
> comma.
> > The
> > > range is 1 to 4094.
> > >
> > > cost
> > > Path cost. The range is 1 to 200000000, with
> > higher
> > > values meaning higher costs.
> > >
> > >
> > >
> > >
> > > Defaults
> > > The default path cost is computed from the
> > interface
> > > bandwidth setting. These are the IEEE default
> path
> > > cost values:
> > >
> > > 1000 Mbps4
> > >
> > > 100 Mbps19
> > >
> > > 10 Mbps100
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> >
>
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12235se/cr/cli3.h
> > tm#wp1945601
> > >
> > >
> > >
> > > --- nhatphuc <nhatphuc@gmail.com> wrote:
> > >
> > > > Hi Group,
> > > >
> > > > Can you give me the link to "spanning-tree
> > pathcost
> > > > method" command of 3550
> > > > or 3560? I've surfed the CiscoDoc but hasn't
> > found.
> > > >
> > > > Thanks
> > > >
> > > > Phuc
> > > >
> > > >
> > >
> >
>
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