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Beginner

Questions about STP.

Hello All,

 

i always find STP very confusing , however i am currently studying switching and i have the below questions:-

 

1-i believe that the root switch election will occur before the ports goes to any state , is that true?

 

2-why would the root bridge determine the hello interval for the nonroot switches if they will just relay the BPDUs and wont send any hello anyway?

 

3-if we have 5 Switches as follows, 1>2>3>4>5 , lets say all switches connected to each other by 1 link and no redundancy and switch 5 is the root , in order for switch 2 to talk to switch 1 , should it forward first the frame out of it's root port to reach the root then the root switch send this frame back to 1? or it will forward directly to switch 1? and does a reference point
means that it will determine the blocked and DP ports for all switches? if yes then what does a central hub means?

from the OCG "The root bridge election is based on the idea that one switch is chosen as a common reference point, and all other switches choose ports that have the best-cost path to the root. The root bridge election is also based on the idea that the root bridge can become a central hub that interconnects other legs of the network. Therefore, the root bridge can be
faced with heavy switching loads in its central location."

 

4-i have a real switch and i have performed the debug spanning-tree switch status command , i noticed once the the port came up , it's goes directly to listening and skipped the blocking state ( 20 seconds) , why did it do that? and how did they switch know it's going to be a DP however this port roles is determined during the listening state?? i have this quote from the OCG "Because this port was eligible as a root port, the show command never could execute fast enough to show the port in the Blocking state"

4 REPLIES 4
Enthusiast

Re: Questions about STP.


1. there are four state in STP, blocked, listening, learning and forwarding. Root and port election will start in Listening state.
2. STP goal is to eliminate loops. At the very beginning all switches claimed to be Root and send BPDUs but only 1 can win. After election, only root sends BPDUs and dictates other settings.
3. BPDUs, Root sw, STP ports are there to find redundant paths and eliminate possible loops. Yes, BPDUs will flow from Root sw downstream to other switches every 2 seconds but this does not mean that User data/traffic will flow same direction. Traffic can flow from 1 to 2 only if that is shortest path and ports are open (non-blocking).
4. Yes, port roles is determined during the listening state. There is a brief blocking state when port comes up but I never seen it.   see my replies and doc in https://community.cisco.com/t5/switching/switch-still-send-bpdu-in-whole-listening-state/td-p/3906335

 

Regards, ML
**Please Rate All Helpful Responses **

Beginner

Re: Questions about STP.

thank you a lot.
Beginner

Re: Questions about STP.

i will check the thread you have provided because something still confusing :D , if the root bridge and port roles will be determined in the listening state, so why the blocking state has been skipped and how the switches knew that those ports are eligible to become root or designated ports ?

Enthusiast

Re: Questions about STP.


blocking port does not send BPDUs, u need exchange BPDUs in the listening state for Root and port elections. after Root election, Root sends BPDUs with Cost of 0, neighboring switches add its cost to BPDU and pass it over downstream. RP/DP ports are elected based on total cost to reach Root, if equal cost, lowest sender Bridge ID wins; if still a tie, lowest sender port id.
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