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Bridge Spanning Tree 3-19
NB-30 Bridging
Bridge Level Parameters
Bridge Priority
This Þeld displays the ÒpriorityÓ component of the NB-30Õs unique bridge
identiÞer. The Spanning Tree Algorithm assigns each bridge a unique identiÞer,
which is derived from the bridgeÕs MAC address and the Priority. The bridge with
the lowest value of bridge identiÞer is selected as the Root. A lower priority
number indicates a higher priority; a higher priority enhances a bridgeÕs chance
of being selected as the Root.
You can edit this text box to change network topology, if needed. The default
value is 8000.
Root Priority
The ÒpriorityÓ component of the unique bridge identiÞer for the bridge that is
currently the root bridge on the network.
Root Bridge
Displays the MAC address of the bridge that is currently functioning as the Root
Bridge.
Root Cost
Indicates the cost of the data path from this bridge to the Root Bridge. Each port
on each bridge adds a ÒcostÓ to a particular path that a frame must travel. For
example, if each port in a particular path has a Path Cost of 1, the Root Cost
would be a count of the number of bridges along the path. (You can edit the Path
Cost of bridge ports as described later.) The Root BridgeÕs Root Cost is 0.
Root Port
This Þeld displays the identiÞer (the physical index number) of the NB-30 bridge
port that has the lowest cost path to the Root Bridge on the network. If the NB-30
is currently the Root Bridge, this Þeld will read 0.
TIP
Part of a bridgeÕs IdentiÞer is based on its MAC address. In most network installations,
the difference between bridge performance may be negligible. You may, however, Þnd your
data bottle-necked in installations where both a low-performance bridge and a
high-performance bridge are attached to the same LAN segment, and the two (or more)
bridges have the same Priority component set (e.g., at the default 8000 Hex). In such a
scenario you may want to alter the Priority component of the higher performance bridge to
ensure that it becomes root for the segment (or overall root). Remember, if Priority
components are equal, the bridge on the segment with the lowest Hex value of MAC
address would have a better chance of being selected as the root bridge Ñ as it would have
a lower Bridge IdentiÞer. If your bridges come from multiple vendors, they will have
different MAC address values (e.g., Cabletron devices have a lower MAC address than
3Com devices); if they come from the same vendor, the bridge with the earlier manufacture
date will be likely to have the lower MAC address value.