• # Solved (TieBreaker)Sending Port ID the same on more than one port.

Was just curios is to what would STP do in this scenario.
SwitchA is the root bridge.
SwitchA is connected to a hub from Fa1/0/1
SwitchB is connected to same hub from Fa1/0/1,Fa1/0/2, and Fa1/0/3
SwitchA Fa1/0/1 would be Forward Designated Port.

SwitchB would see same Port ID on all 3 ports for the tiebreaker. Which ports would be blocking and which would be the Forwarding Root Port?

I assume the network would just go mental but either or I am still curious.

• OK, I had to diagram this out before I could see:

So a single hub, 2 switches

-- Switch A has a single link to the hub, you've marked it as the `Root Bridge`
-- Switch B has a 3 links (fa1/0/1, fa1/0/2, and fa1/0/3) it's is then by default the `Non-Root Bridge`
-- Each segment cost to the hub If you break it down to the 10Mbps (default for most hubs) let's say it's 100. For each of the 3 connections then the `cost` is all equal to the root bridge.

• Select the port connected to the path with the lowest accumulated Spanning Tree Path Cost to the Root Bridge as the Root Port, when a Non-Root Switch has multiple paths to reach the Root Switch. << ALL COST ARE EQUAL -- NO DECISION>>

• If COSTS are equal, select the port connected to the neighbor switch which has the lowest Switch ID value as the Root Port. <<ONLY ONE SwitchID --NO DECISION>>

• If one SwitchID, then Non-Root Switch will select the local port which receives the lowest port Spanning Tree Port Priority value from neighbor Switch as the Root Port. <this would be equal as well for each link on Switch B -- NO DECISION>>

• If the received Spanning Tree Port Priority value values are the same between the connecting ports to reach the Root Bridge, Non-Root Switch will select the port which receives the lowest physical port number from neighbor Switch as the Root Port. This is the last tie breaker.

So the result is this... Switch B will designate fa1/0/1 as the `Root Bridge`. Because a single switch (Switch can have only ONE Root Bridge, then the others will be designated NDP (Non-Designated Port Role and it's state will be Blocking (Alternative) depending on version of STP you're running.

Cordially,
Ronnie Wong
Edutainer Manager, ITProTV

*if the post above has answered the question, please mark as solved.
**All "answers" and responses are offered "as is" and my opinion. There is no implied service, support, or guarantee by ITProTV.

• Thanks Ronnie for the reply.
So I made a diagram, lets see if I can get it on here.

"Non-Root Switch (SwitchB) will select the port which receives the lowest physical port number from neighbor Switch (SwitchA) as the Root Port. This is the last tie breaker.

In my scenario SwitchB would get the same lowest physical port number from its neighbor on all 3 of its interfaces.

• Ok lets try this
https://1drv.ms/i/s!Ap0wl_pv96BHg1BHJQydp6h-4pLW

I think the insert picture thing on this forum does no work.
Doesn't seem to work for me anyhow.

• Ok so here's what happens, I actually drew what you sent too. When you get to this point and it can't do all of the above. The other tiebreaker that can kick in as well continues with the lowest MAC address on the Switch B ports that connected. Each switchport on SwitchB will have a MAC address associated with it and the lowest MAC address will be associated with the lowest port number, usually.

Cordially,
Ronnie Wong
Edutainer Manager, ITProTV

*if the post above has answered the question, please mark as solved.
**All "answers" and responses are offered "as is" and my opinion. There is no implied service, support, or guarantee by ITProTV.

• Oh that's too cool.
Thanks again Ronnie much appreciated.

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