>From the CISCO web site.
http://www.cisco.com/warp/public/473/5.html#oper
Rules of Operation - STP works as follows: When the switches first come up,
they start the root switch selection process by each switch transmitting
BPDU to its directly connected switch on a per-VLAN basis.
As the BPDU goes out through the network, each switch compares the BPDU it
sent out to the one it received from its neighbors. From this comparison,
the switches come to an agreement as to who the root switch is. The switch
with the lowest priority in the network that wins this election process.
(Remember, there will be one root switch identified per VLAN.) After that
root switch has been identified, the switches follow the rules defined
below:
STP Rule One: All ports of the root switch must be in forwarding mode
(except for some corner cases where self-looped ports are involved).
Next, each switch determines their best path to get to the root. They
determine this path by comparing the information in all the BPDUs received
on all their ports. The port with the smallest information contained in its
BPDU is used to get to the root switch; that port is called the root port.
After a switch figures out its root port, it proceeds to Rule Two.
STP Rule Two: Once a switch determines its root port, that port must be set
to forwarding mode.
In addition, for each LAN segment, the switches communicate with each other
to determine which switch on that LAN segment is best to use for moving data
from that segment to the root bridge. This switch is called the designated
switch.
STP Rule Three: In a given LAN segment, the designated switch's port that
connects to that LAN segment must be placed in forwarding mode.
STP Rule Four: All other ports in all the switches (VLAN-specific) must be
placed in blocking mode. This is only for ports that are connected to other
bridges or switches. Ports connected to workstations or PCs are not affected
by STP; they remain forwarded.
FOR a Look at path cost you might take a look at
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3ks/3000/aicfgcsl.htm
#11023
-----Original Message-----
From: Tom Pruneau [mailto:[EMAIL PROTECTED]]
When switch send spanning tree updates those updates are sent in BPDUs
(bridge protocol data units). The BPDU will have a source mac address
associated with the originating switch/VLAN number. If a switch recieves
multiple BPDUs that indicate the same root cost it will pick the one which
came from the switch with the lowest (I'm pretty sure it's lowest and not
highest but I may be wrong) MAC address. There is also a port priority
which I believe (I'm not sure) can be configured to aid in the selection of
the root port
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