Got it

STP election

Created: Apr 22, 2020 11:34:55Latest reply: Apr 22, 2020 11:36:46 104 1 0 0 0
  Rewarded HiCoins: 0 (problem resolved)

Hello everyone,

The topology and configuration are as follows:

1

[sw1]vlan batch 10 20

[sw1]stp enable

[sw1]int g0/0/1

[sw1-GigabitEthernet0/0/1]port link-type trunk

[sw1-GigabitEthernet0/0/1]port trunk allow-pass vlan all

[sw1-GigabitEthernet0/0/1]int g0/0/2

[sw1-GigabitEthernet0/0/2]port link-type trunk

[sw1-GigabitEthernet0/0/2]port trunk allow-pass vlan all

[sw1]stp root primary  //Setting the root switch 

[sw2]vlan batch 10 20

[sw2]stp enable

[sw2]int g0/0/1

[sw2-GigabitEthernet0/0/1]port link-type trunk

[sw2-GigabitEthernet0/0/1]port trunk allow-pass vlan all

[sw2-GigabitEthernet0/0/1]int g0/0/3

[sw2-GigabitEthernet0/0/3]port link-type trunk

[sw2-GigabitEthernet0/0/3]port trunk allow-pass vlan all

[sw2]stp root secondary

[sw2-GigabitEthernet0/0/1]stp cost 5 

[sw3]vlan batch 10 20

[sw3]stp enable

[sw3]int g0/0/1

[sw3-GigabitEthernet0/0/1]port link-type trunk

[sw3-GigabitEthernet0/0/1]port trunk allow-pass vlan all

[sw3-GigabitEthernet0/0/1]int g0/0/3

[sw3-GigabitEthernet0/0/3]port link-type trunk

[sw3-GigabitEthernet0/0/3]port trunk allow-pass vlan all

Check the port status of each switch

1

2

3

The cost of GE 0/0/1 on SW2 is set to 5. Why is GE 0/0/1 still the root port? G0/0/3 should be the root port. In addition, G0/0/3 of SW3 should be enabled. Why is the red mark on the diagram the same as the result?

Please help me! Thank you!


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Featured Answers
DDSN
Admin Created Apr 22, 2020 11:36:46

Hi WDNJSQ,

In this experiment, setting the cost to 5 does not change the port role. To change the port role, set the cost calculation mode to stp pathcost-standard legacy and set the cost to a value greater than the sum of the costs of the other links. 

In STP election, BIDs are compared first. BIDs consist of priorities and MAC addresses. If no priority is specified, the priority is 32768. MAC addresses are compared before costs are compared. 

The following figure shows the default port roles.

1

2

3

Change the cost value of g0/0/1 on SWB.

4

The port role changes.

5

I hope it helps!


View more
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All Answers
DDSN
DDSN Admin Created Apr 22, 2020 11:36:46

Hi WDNJSQ,

In this experiment, setting the cost to 5 does not change the port role. To change the port role, set the cost calculation mode to stp pathcost-standard legacy and set the cost to a value greater than the sum of the costs of the other links. 

In STP election, BIDs are compared first. BIDs consist of priorities and MAC addresses. If no priority is specified, the priority is 32768. MAC addresses are compared before costs are compared. 

The following figure shows the default port roles.

1

2

3

Change the cost value of g0/0/1 on SWB.

4

The port role changes.

5

I hope it helps!


View more
  • x
  • convention:

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