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Spanning Tree Overview Configuring Spanning Tree Parameters
page 5-8 OmniSwitch 6600 Family Network Configuration Guide April 2006
The following diagram shows the logical connectivity of the same physical topology as determined by the
Spanning Tree Algorithm.
Active Spanning Tree Topology Example
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 7700
TM
OmniSwitch 7700
TM
(Root Bridge)
(Designated Bridge)
2/1
2/10
2/2
2/3
3/10
3/12/9
3/8
Bridge ID
Bridge ID
Bridge ID
Bridge ID
PC=19
3/2
Forwarding
Blocking
Root Port
Designated Port
3/9
10, 00:00:00:00:00:01 13, 00:00:00:00:00:04
12, 00:00:00:00:00:0311, 00:00:00:00:00:02
PC=19
PC=100
PC=4
PC=19
Path Cost
PC
Switch C
Switch A
Switch D
Switch B
OmniSwitch 6648
OmniSwitch 6648
OmniSwitch 6648
OmniSwitch 6648