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Bridging
5-16 Bridge Spanning Tree
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) of the selected
module’s bridge port that has the lowest cost path to the Root Bridge on the
network. If the selected module is currently the Root Bridge, this field will read 0.
Protocol
Displays the Spanning Tree Algorithm Protocol type the selected module is
currently using. The choices are:
• 802.1
• DEC (DEC Lanbridge 100)
• None
The following four fields display values used for various Spanning Tree timers
that are set at the Root Bridge and this bridge. In Spanning Tree operations, the
value used for the tree is the one set at the Root Bridge (with the exception of
Hold Time, which is a fixed value); but you can change the value for each bridge
on your network in the event that it becomes Root.
TIP
Part of a bridge’s Identifier is based on its MAC address. In most network installations,
the difference between bridge performance 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.