Quick STP question on blocked port

[Jerome Vidal]

Hi Folks,

I'm trying to gain a better understanding of the STP process. I'm all clear on root bridge, root ports, designated ports but I'm confused about the block ports (alternate port). I've attached a screen capture of an STP topology showing that f0/1 on SW3 will be the blocked port (AP/alternate port) whereas f0/1 on SW1 will be the designated port (DP). Could somebody please advise what influences f0/1 on SW3 to be a blocked port (AP) in this scenario?

[Julio E. Moisa]

Hi

As S2 is the root bridge, it will configure its ports like designated ports, so in this case it has 3 designated ports, to S1, to S3 and to S4. 

As you can have only one designated port at the segment, in the case between S1 and S3 it will elect the designated port with lowest cost to the root. In this case a Giga (4) port has lowest cost than a Fast (19) that is the reason why F0/1 on S3 is blocked.

https://www.google.com.sv/search?q=spanning+tree+interface+costs&rlz=1C1HIJA_enSV704SV704&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjMic_WodbTAhXEYyYKHd_KAsEQ_AUIBigB&biw=1517&bih=735#imgrc=j5l0613olIczHM: 

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:-)

[Jerome Vidal]

Hi Julio,

Thanks your feedback! I understand that a Giga (4) port has lowest cost than a Fast (19) but both switches (S1 & S3) do have Giga and Fast so I still don't get why f0/1 on SW3 is blocked, rather than f0/1 on SW1.To me I don't see any difference between S1-S2 path and S3-S2 path but I would highly appreciate if you can pinpoint any difference.

[Julio E. Moisa · Accepted solution]

Hi

Apologies, I didnt see the S1's port was Fast as well, I agree with Rolf explanation.

[Rolf Fischer]

Jerome,

(...) showing that f0/1 on SW3 will be the blocked port (AP/alternate port) whereas f0/1 on SW1 will be the designated port (DP). 

The desicion is made by comparing the BPDUs of both ports on that segment. We can assume that the pathcost to the rootbridge is the same, so the next step in the decision process is determine the lower (sender) Bridge ID. The configurable part of the BID is the priority, which is 32.769 (default) on both Sw1 and Sw3. So the tie-breaker in this case is the MAC address (non-configurable part of the BID): Sw1 has the lower MAC address, so Fa0/1 on Sw1 becomes the DP for this segment. Fa0/1 on Sw3 is neither the RP of the switch nor the DP of the segment, thus it will result in blocking state.

HTH
Rolf

[Jerome Vidal]

That is such a great explanation! Please feel free to share a more complex STP topology if you can. Thanks a lot!

[Rolf Fischer]

You're welcome.

The attached topology requires a slightly deeper dive into the STP decision process. The configurable parts of Bridge- and Port IDs are red, the fixed parts black. Hope you'll enjoy it ;)