STP question. How is the BLK port determined?

mzinz

I thought that the switchport with the _higher_ MAC address was the one that blocked a link that would cause a loop. When testing this out in my lab, it seemed like I was getting mixed results. Does priority play into this as well?

aordal

It's all about the BRIDGE ID (BID) which is a combination of Priority and MAC. MAC only comes into play if the priority's are all equal.

mzinz

aordal wrote: »

It's all about the BRIDGE ID (BID) which is a combination of Priority and MAC. MAC only comes into play if the priority's are all equal.

Ok. So in the following scenario:

If two switches detect a loop (suppose there are two physical connections and no etherchannel). And both links are the same speed (same cost), one link must be blocked. In this case, how exactly would the blocked link be determined, and which switch would BLK, and which one would FWD/Alternate?

Ports could be as shown below:

SW1-(FA0/1)

---

(FA0/12)-SW2
SW1-(FA0/2)

---

(FA0/13)-SW2

Switch1 MAC: 0001.0001.0001
Switch2 MAC: 0002.0002.0002

Forsaken_GA

Is priority set to default or has it been modified on either switch? If so, how?

I understand the scenario you're asking, but there's not enough information provided to be able to make a definitive determination without some assumptions in play.

And assumption is the mother of all <bleep>

networker050184

The lowest port ID would be the most preferable. So in your little drawing switch 2 port 13 would block assuming all settings are default.

kryolla

since the priority are the same it will be up to mac to determine who is root bridge. SW1 will be elected root and all ports will be forwarding and designated ports. Sw2 will now need to make a decision what port is root and what port is designated. The BPDU sw2 receives will be favorable on port 12 because of SW1 port so that will be the RP. Now you have two DP on the same segment and there can only be 1. Since SW1 has a better BPDU it will be forwarding and sw2 port 13 will be blocking as long as it is receiving a better BPDU. Remember RP receives BPDU then it adds it own info such as cost and priority then sends it out DP.

mzinz

Thank you for all the responses.

And yes, we can assume all priorities are defaulted.

So in my scenario, SW1 is elected root (Lower MAC), and SW2 will have to block a port because it has two DP (I understood this).

SW2 then has to make a decision of what port to block. *To make this decision, it uses port priority and port ID. Priority is at default (no port-priority command specified), so it blocks the port with the highest port ID*

Just talking out loud Thanks again.

mzinz

kryolla wrote: »

The BPDU sw2 receives will be favorable on port 12 because of SW1 port so that will be the RP. .

I know I'm really overanalyzing this thread, but I want to make sure I understand every tiny aspect of this

In your quote above, you made it sound like SW2 PORT12 will be the RP because of something related to SW1... It was my understanding that SW2 will be blocking whichever port has the higher local PORT ID (because no port-priority is specified), is this not correct?

kryolla

Try to connect Sw1 port 1 to Sw2 port 13 and see if that becomes the RP in Sw2. Now connect it back to port 12 and play with the port priority command on Sw1 to make port 2 more favorable and see if port 13 Sw2 becomes RP.

mzinz

kryolla wrote: »

Try to connect Sw1 port 1 to Sw2 port 13 and see if that becomes the RP in Sw2. Now connect it back to port 12 and play with the port priority command on Sw1 to make port 2 more favorable and see if port 13 Sw2 becomes RP.

Perfect test Thank you for the guidance.