Cabletron Systems 9H42X-XX

Bridge Spanning Tree 5-13

Bridging

Bridge-level Parameters

The Bridge Spanning Tree window provides the following bridge-level

information:

Bridge Priority

This ﬁeld displays the “priority” component of the 9H42x-xx module’s unique

bridge identiﬁer. The Spanning Tree Algorithm assigns each bridge a unique

identiﬁer, which is derived from the bridge’s MAC address and the Priority. The

bridge with the lowest value of bridge identiﬁer is selected as the Root. A lower

priority number indicates a higher priority; a higher priority enhances a bridge’s

chance of being selected as the Root.

You can edit this text box to change network topology, if needed. The default

value is 8000 (hex value 80-00).

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 “cost” 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 Bridge’s Root Cost is 0.

Root Port

This ﬁeld displays the identiﬁer (the physical index number) assigned to the

bridge port that has the lowest cost path to the Root Bridge on the network. If the

9H42x-xx module itself is currently the Root Bridge, this ﬁeld will read 0.

TIP

Part of a bridge’s Identiﬁer is based on its MAC address. In most network installations,

the difference in performance levels between bridges may be negligible. You may, however,

ﬁnd 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 — as it would have

a lower Bridge Identiﬁer. 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.