From: Ash (nester2k@gmail.com)
Date: Thu Oct 04 2007 - 02:54:10 ART
thats correct...Cost is added as the BPDU's are accepted on a port and no
addition is made as those BPDU's are passed to the next switch.
Slevin,
To answer your question, consider the root port election criteria
a) Lowest pathcost to the root
b) Lowest Sender Bridge ID
c) Lowest Port ID ( Lowest Port Pri and if there's a tie, then the lowest
Port id)
As you can see from the above list, Lowest path cost to the root is what
initially determines if a port will become the root Port. Only after there's
a Tie between two equal cost paths to the root does the Sender BID and PORT
Priority come into Play.
In a 3 switch envrionment, this TIE by default is formed when a switch has
redundant non-etherchannel trunk connections to the ROOT Switch itself
(provided they are all Same SPEED links and/or the cost on the links has
been tweaked such that an artifical tie is created). Now that there's a tie,
the sender bid will come into play which really is the same since your
directly connected to the root switch itself. This leaves you with PORT
priority as the last tie breaker. At this stage, you can change the PORT
priority on the root switch to influence the root port election on the non
root switch.
Since you have Etherhcannel links between the 3 switches which are treated
as single links in terms of Spanning tree, port priority on the root can't
be used to modify the layer2 forwarding path of the MSTP instances in this
scenario. The only way would be to increase the port cost of the MST
instance on the non preferred port towards the root allowing a better BPDU
to be received on the preferred port through the redundant path.
rgrds,
On 10/3/07, Bob Sinclair <bob@bobsinclair.net> wrote:
>
> Joseph Brunner wrote:
> > As a student of Brian's VOL II. Workbook dojo I learned we change stp
> PORT
> > PRI to influence root path selection going downstream (root to leaf),
> while
> > we influence COST to influence root path selection going upstream (leaf
> to
> > root). Just remember COST is added OUT of a port as the BPDU leaves the
> > bridge its added.
> >
> >
> Hi Joseph,
>
> Absolutely right about using cost to influence root port selection, but
> COST is added as the BPDU arrives, not as the BPDU is sent. I would
> refer you to Kennedy Clark's Cisco LAN Switching page 171. Or you
> could connect a sniffer to a root bridge and see the cost in the BPDU is
> zero.
>
> HTH,
>
> --
>
>
> Bob Sinclair CCIE 10427 CCSI 30427
> www.netmasterclass.net
>
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