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Tie Breakers in STP

Hi again,

I thought tie breakers in STP were MAC address and bridge priority but in the following question, the correct answer are A and C.

Question: When electing a root port for a nonroot switch, which values are used to break ties if required?

A. Upstream BID
B. MAC address
C. Port ID
D. Priority

Correct answer: A C

Can anyone give me a short explanation?

Thank you

9 REPLIES 9
chris noon
Beginner

This question is referring to normal switches in the topology and not the root bridge.  It is asking how these switches work out the quickest/shortest path to the root bridge.  In this instance it uses the BID and Port ID.

MAC address and Priority are used in the election of the root bridge.

Thank you, chris,

So, root ports are always elected according to BID and Port ID? And only these?

Hi Jose, 

You might enjoy this quick read on STP non-root switches, path selection:

http://www.omnisecu.com/cisco-certified-network-associate-ccna/how-spanning-tree-protocol-stp-select-root-port.php

However, to answer your question the non root switches use a number of variables to calculate the path the the root switch, in order:

1) The lowest cost to the root bridge.

2) Lowest neighboring switch ID.

3) Lowest priority value from neighboring switch.

4) Lowest neighboring port ID.

Thanks for that great link. I didn't know that website, but now it's marked as favourite...

Excuse me, I don’t understand how the third paragraph uses priority value from neighboring switch, if it was used in the second paragraph. Is the BID not priority + MAC or is it meant PORT priority value from neighboring switch?
ahakels
Beginner

This step comes after the root Bridge election.

STP calculates the path cost based on the media speed (bandwidth) of the links between switches and the port cost of each port forwarding frame. Spanning tree selects the root port based on the path cost. The port with the lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state.

If the speed/duplex of the port is changed, spanning tree recalculates the path cost automatically. A change in the path cost can change the spanning tree topology.

Each switch determines the best path to get to the root. The switches determine this path by a comparison of the information in all the BPDUs that the switches receive on all ports. The switch uses the port with the least amount of information in the BPDU in order to get to the root switch; the port with the least amount of information in the BPDU is the root port. After a switch determines the root port, the switch proceeds.

For more info:

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

Thanks ahakels,

I knew that the root port was elected depending on its path cost but I thought that this cost was calculated according to MAC address and Priority instead of Upstream BID and Port ID.

jetmelt123
Beginner

This is what I hate about cisco questions. They are so convoluted! Mac addresses ARE used to choose the root port! Priority IS used to choose the root port! port ID is not used but the port id's priority is used! Upstream BID? Who defines upstream?

And dont tell me from wence conf bpdus come. I know that! Maybe my root switch is downstream in my network! It is in fact. Downstream! Thats right! At the bottom of my topology. 

How about straight forward exam questions.

Here are four legit answer choices

A.) Lowest Recieved Port Priority : confusing with (port id) (priority)

B.) Lowest Recieved Port Number

C.) Lowest Recieved Bridge ID : confusing with (Mac address) (upstream BID)

D.) Lowest Path Cost

FFS The tests are timed as well and we have to figure out your screwy semantics?

gornication
Beginner

So what to do with such questions?
I think the answer should be one, but it is not clear at what stage the tie came about.
Why upstream? The decision is made by a nonroot switch, for which all incoming BPDUs are from somewhere above.
What does priority in clause D refer to?

Is the problem in the test or in me?