Spanning tree protocol

General
1

Sir can you explain me if 2 switches are connected with redundant links then which port will be blocked.And how?
Interviewer asked me this question.

2

Sir please explain me

3

Hi @Jaspreet As per my knowledge, in normal scenario, The switch with lowest mac address will be the root Bridge where none of the port will be in block state. And the other switch with the highest interface port number will be in block state. Example Switch No. 2 = has two ports 0/1 & 0/2 so the 0/2 will be blocked by default.

Correct me if i am wrong.

4

Thanks for the response Ashwini

5

For this, you need to know the steps of election for STP

Steps for election:

  1. One switch is elected as the root bridge. All ports on the root bridge are designated ports (forwarding state). Root bridge selection:
    1: Lowest bridge ID

  2. Each remaining switch will select ONE of its interfaces to be its root port (forwarding state). Ports across from the root port are always designated ports.
    Root port selection:
    1: Lowest root cost
    2: Lowest neighbor bridge ID
    3: Lowest neighbor port ID

  3. Each remaining collision domain will select ONE interface to be a designated port (forwarding state). The other port in the collision domain will be non-designated (blocking)
    Designated port selection:
    1: Interface on switch with lowest root cost
    2: Interface on switch with lowest bridge ID
    Non Designated Port selection:
    1: Highest Port ID

6

Stp election I know.
But if redundant links connect between 2 switches then which port will be blocked.
Which interface will be block?

7

Atul sir can you please give me the answer of above question

8

Hi Jaspreet,
Sometimes it will be Good to Post an image here so that we can explain for you, looking at your image/photo

and of-course for you to understand. you need to watch one of Atul s Video regarding STP.

9

image
image870×221 5.32 KB

This Scenario is based on Port Priority:

Procedure

  1. Determine who is the Root Bridge. comparing the lowest mac-address. lowest become root-Bridge
  2. The Root bridge will be a upstream switch. and all ports are in forwarding state.
  3. So check, in the root Bridge which port is the highest ? F0/1 or F0/2 ? F0/2
  4. So the opposite of the highest port from root bridge will be block. so in simple terms a port that is
    facing port f0/2 from non-root bridge will be block.

Another name for Switch is Bridge

10

Thanks Simon.
Now got it