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Beginner

STP Tie Breaker: lowest BID

Hey guys,

I'm learning about STP at the moment and am a bit confused about the tie breakers it's using to calculate the best path to the root bridge.

As we all know STP uses:
1. lowest BID

2. lowest root path cost

3. lowest sender BID

4. lowest sender Port ID

Now my question is: when will it ever come to the situation that I will have a tie on the lowest BID? Since the MAC address is part of the BID there will never be the situation that the BID will be exactly the same on 2 Switches, or am I missing out on something here?

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VIP Advisor

Re: STP Tie Breaker: lowest BID

here is the reference for more clarity :

 

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/spantree.html#wp1174063

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4 REPLIES 4
Advocate

Re: STP Tie Breaker: lowest BID

Hi @Manonym ,

 

As you may know, the BID is made up of the Bridge Priority and the MAC address.

Then, the switch with the lowest Bridge Priority will win in a tie.

If both switches have the same Bridge Priority, then the switch with the lowest MAC address will win.

As you probably know, there are no two equal MAC addresses, so there would always be a difference that indicates a winner of the tie.

 

Regards

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VIP Advisor

Re: STP Tie Breaker: lowest BID

here is the reference for more clarity :

 

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/spantree.html#wp1174063

BB
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Beginner

Re: STP Tie Breaker: lowest BID

Thank you for the link.
I was a bit confused and mixed up the "Root Bridge election" vs. "choosing the best path to the root bridge" but now I got it!
Hall of Fame Expert

Re: STP Tie Breaker: lowest BID

Hello,

 

>> Now my question is: when will it ever come to the situation that I will have a tie on the lowest BID?

for point 3 Sender Bridge ID a tie is possible if a downstream switch has two different ports not aggregated in a bundle connected to the same upstream switch.

Another case when the sender Bridge ID is the same is when you connect with a cable two ports of the same switch. In that case the port with the highest port-id will go in blocking / discarding state (in first STP / Rapid STP)

 

Hope to help

Giuseppe

 

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