Spanning Tree Algorithm
110
Instruction Manual - NXA-ENET8-POE+
Administrative path cost cannot be used to directly determine the root port on a switch. Connections to other devices use IEEE
802.1Q-2005 to determine the root port as in the following example.
For BPDU messages received by i1 on SW3, the path cost is 0.
For BPDU messages received by i2 on SW3, the path cost is that of i1 on SW2.
The root path cost for i1 on SW3 used to compete for the role of root port is 0 + path cost of i1 on SW3; 0 since i1 is directly
connected to the root bridge.
If the path cost of i1 on SW2 is never configured/changed, it is 10000. Then the root path cost for i2 on SW3 used to compete for
the role of root port is 10000 + path cost of i2 on SW3.
The path cost of i1 on SW3 is also 10000 if not configured/changed. Then even if the path cost of i2 on SW3 is configured/
changed to 0, these ports will still have the same root path cost, and it will be impossible for i2 to become the root port just by
changing its path cost on SW3.
For RSTP mode, the root port can be determined simply by adjusting the path cost of i1 on SW2. However, for MSTP mode, it is
impossible to achieve this only by changing the path cost because external path cost is not added in the same region, and the
regional root for i1 is SW1, but for i2 is SW2.
Perform these steps to configure interface settings for STA:
1.
Click
Spanning Tree
>
STA
.
2.
Select
Configure Interface
from the Step list.
3.
Select
Configure
from the Action list.
4.
Modify any of the required attributes.
5.
Click
Apply
.
Default STA Path Costs
Port Type
Short Path Cost
(IEEE 802.1D-1998)
Long Path Cost
(IEEE 802.1D-2004)
Ethernet
65,535
1,000,000
Fast Ethernet
65,535
100,000
Gigabit Ethernet
10,000
10,000
10G Ethernet
1,000
1,000
FIG. 117
Determining the Root Port
FIG. 118
Configuring Interface Settings for STA
SW1
i2
i1
i1
i2
i2
i1
SW2
SW3