
Using Transparent Bridge Services
114060 Rev. A 1-11
In constructing a loop-free topology, the bridges within the extended network
follow these steps:
1. Elect a root bridge.
The bridge with the lowest priority value becomes the root bridge and serves
as the root of the loop-free topology. If priority values are equal, the bridge
with the lowest bridge MAC address becomes the root bridge.
2. Determine path costs.
The path cost is the cost of the path to the root bridge offered by each bridge
port.
3. Select a root port and elect a designated bridge on each LAN.
Each bridge designates the port that offers the lowest-cost path to the root
bridge as the root port. In the event of equal path costs, the bridge examines
the paths’ interfaces to the root bridge. The port (interface) of the path with
the lowest interface priority to the root bridge becomes the root port. For
example, Figure
1-9 shows how Bridge A determines its root port.
Figure 1-9. Root Port Determination (Equal Path Costs)
If the paths’ interfaces to the root bridge are also equal, then the root port is
the port on the bridge with the lowest priority value (Figure
1-10).
LAN A
IF 3
IF 1
IF 1 = priority 1
IF 2 = priority 2
IF 4
LAN B
IF 2
IF 3 = priority 3
IF 4 = priority 4
Path costs from IF 3 to root bridge and
from IF 4 to root bridge are equal.
---------AND--------
IF 3 becomes root port because
IF 1’s priority is lower than IF 2’s
priority.
Root
Bridge
Bridge A
Given:
Then:
BRG0028A
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