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Please show the process of Root Port Election in STP?

author
Ved khanna
,
IT Manager
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Can You answer this question?
author
Mr. Rajat Goyal

- ROOT PORT Election Criteria on the basis of information in BPDUs 

1. Lowest Cost on interfaces towards the Root Bridge.

2. Lowest Sender Bridge ID.

3. Lowest Sender Port ID.

- In this topology, since all the switches are having the same bridge priority, according to lowest Mac address election criteria, SW1 will become Root Bridge.

- SW2 is getting BPDU with the cost of 0 from Root bridge on interface e0/0, similarly SW3 is getting BPDU with the cost of 0 from Root Bridge on interface e0/0

- SW2 is forwarding BPDU with the cost of 100 through interface e0/1 towards SW3 which it is receiving on interface e0/1.

- SW3 is forwarding BPDU with the cost of 100 through interface e0/1 towards SW2 which it is receiving on interface e0/1.

- Now both switch will compare the cost to reach Root Bridge through the interface on which they have received the least cost.

- SW2 BPDU on interface e0/0, cost = 0 | SW2 BPDU on interface e0/1, cost = 100

- SW3 BPDU on interface e0/0, cost = 0 | SW3 BPDU on interface e0/1, cost = 100

- Whichever the interface is have the least cost to reach Root Bridge will be elected as Root Port

- In this Topology, e0/0 on SW2 will become Root Port and e0/0 on SW3 will become Root Port.

- In this topology, since all the switches are having the same bridge priority, according to lowest Mac address election criteria, SW1 will become Root Bridge.

- SW2 is getting BPDU with the cost of 0 from Root bridge on interface e0/0, similarly SW3 is getting BPDU with the cost of 0 from Root Bridge on interface e0/0 and e0/2

- SW2 is forwarding BPDU with the cost of 100 through interface e0/1 towards SW3 which it is receiving on interface e0/1.

- SW3 is forwarding BPDU with the cost of 100 through interface e0/1 towards SW2 which it is receiving on interface e0/1.

- Now both switch will compare the cost to reach Root Bridge through the interface on which they have received the least cost.

- SW2 BPDU on interface e0/0, cost = 0 | SW2 BPDU on interface e0/1, cost = 100

- SW3 BPDU on interface e0/0, cost = 0 & e0/2, cost = 0 | SW3 BPDU on interface e0/1, cost = 100

- Here on SW3 there are two links which are getting the same cost from Root Bridge

- Sender Bridge ID in this case is also same which is aabb.cc80.0100

- Sender Port ID in this case on interface e0/0 of SW3 is e0/1 of Root Bridge and on interface e0/2 of SW3 is e0/2 of Root Bridge.

- Now on the basis of third criteria which is Lowest sender Port ID which is e0/1 of SW1 in this case which SW3 is receiving on interface e0/0 will become Root Port

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