Understanding the network.pdf - Back to Home

The root election process is won based on the bridge with the lowest
serial number when normal plug-and-play bridge configuration is
used. However, because the root bridge can set certain behavioral
properties for all the bridges in the network, it is often desirable
(though not always possible) to configure in advance the bridge that
will become the root bridge. On bridges that do support configuration,
it is possible to configure most of the following variables:
• Bridge priority—Adds or subtracts value to the bridge's ID.
• Port priority—Allows one port to be favored over another in
determining which port will be selected as the active port in the
tree. In situations where a bridge (or a switch) has ports with
different bandwidth connected to the same bridge, the higher
bandwidth port will be selected.
• Hello time—Adjusts the period between the generation of BDPU
messages. When adjusting the hello time on a bridge, you must
adjust the hello time on the other bridges on the LAN segments
to the same value.
• Max age—Determines when the bridge's stored BDPU messages
will expire.
• Forward delay—Sets the amount of time the bridge should wait
before forwarding packets on a port. This allows all the other
bridges to run spanning trees and adjust to the change.
• Path cost—A value that is set on each port. It provides the
capability to add additional cost to the root path cost for the
port.
Of all these variables, the bridge and port priority are the most
significant in terms of controlling how the spanning tree will control
itself. If you plan to implement path redundancy, it is wise to take the
time to configure all the bridges to function in such a manner that
ensures a predictable fallover in the case of failure. Alternatively, if
you are not planning to use link redundancy, it is also possible on
most bridges to disable spanning tree, to minimize the amount of
unneeded traffic on your network. This option should be considered
carefully before implementation.
Source Route Bridges
Source route bridging is not subject to the looping and packet
duplication characteristic of transparent bridging, because the
packet's source, not the LAN, determines the packet delivery path.
This method comes at a price, mainly in the form of additional
complexity, because the Layer 2 transmission protocol needs to