Alcatel-Lucent OmniSwitch 6850-48 Network Configuration Manual page 313

Configuring Spanning Tree Parameters
The following diagram shows the logical connectivity of the same physical topology as determined by the
Spanning Tree Algorithm:
Bridge ID
10, 00:00:00:00:00:01
Bridge ID
11, 00:00:00:00:00:02
In the above active Spanning Tree topology example, the following configuration decisions were made as
a result of calculations performed by the Spanning Tree Algorithm:
•
Switch D is the root bridge because its bridge ID has a priority value of 10 (the lower the priority
value, the higher the priority the bridge has in the Spanning Tree). If all four switches had the same
priority, then the switch with the lowest MAC address in its bridge ID would become the root.
•
Switch A is the designated bridge for Switch B, because it provides the best path for Switch B to the
root bridge.
•
Port 2/9 on Switch A is a designated port, because it connects the LAN from Switch B to Switch A.
•
All ports on Switch D are designated ports, because Switch D is the root and each port connects to a
LAN.
•
Ports 2/10, 3/1, and 3/8 are the root ports for Switches A, B, and C, respectively, because they offer the
shortest path towards the root bridge.
•
The port 3/9 connection on Switch C to port 2/2 on Switch D is in a discarding (blocking) state, as the
connection these ports provides is redundant (backup) and has a higher path cost value than the 2/3 to
3/8 connection between the same two switches. As a result, a network loop is avoided.
•
The port 3/2 connection on Switch B to port 3/10 on Switch C is also in a discarding (blocking) state,
as the connection these ports provides has a higher path cost to root Switch D than the path between
Switch B and Switch A. As a result, a network loop is avoided.
OmniSwitch AOS Release 6 Network Configuration Guide
Switch D
(Root Bridge)
2/3
2/2
2/1
PC=19
2/10
2/9
Switch A
(Designated Bridge)
Forwarding
Blocking
Active Spanning Tree Topology Example
Switch C
PC=4 3/8
PC=19 3/9
3/10
PC=100
3/2
PC=19
3/1
Switch B
Root Port
Designated Port
Path Cost
September 2009
Spanning Tree Overview
Bridge ID
13, 00:00:00:00:00:04
Bridge ID
12, 00:00:00:00:00:03
PC
page 13-11