This Spanning Tree Protocol Question Broke Me
Hello guys, it’s Monday again!
Hope you had a smile on your face when you realised that it was Monday, if you didn’t, well, you should begin to reconsider the company you own or work for.
Anyway, I was spending the weekend studying Part IV (IPv4 addressing) of the CCNA & also a bit about REST API on DevNet when my study buddy Mary dropped a question about STP on the discord server.
A David Bombal Question: Which ports in the topology are blocked by Spanning Tree?
At first, figuring out which ports were blocking on S1 & S2 was pretty easy. But then looking at S2 & S3 my brain knocked. Thankfully, Taylor came through. But then a few comments on the solution blew my mind! I didn’t realise this while I was studying the OCG.
1. Tie Breakers are Hierarchical: So, I knew all the tie-breakers at first, but then, I would randomly pick them while making comparisons, I was wrong. The tie-breakers are as follows:
Lowest Root Path Cost
Lowest SENDING Bridge ID
Lowest Configured Port Priority
The lowest SENDING interface number
💡 Also, as usual, the lowest value wins.
2. When Comparing Ports Only Compare Sending Ports: So, while solving for which ports are blocking on S1 vs S12, one wouldn’t realize this rule since S1 is the root switch. But then, while solving for which ports are blocking on S2 vs S3 this rule comes in pretty handy.
SOLVING FOR BLOCKING PORTS on S2 vs S3
So, S2 is the sending switch (not “sending” as in regarding frames, but regarding STP Hello BPDUs). Hence, we will be solving with the sending interfaces which are (Gi0/2 & Gi0/3).
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We compare the lowest root path cost = Equal 🏆
We compare the lowest sending bridge ID = Equal (the two ports are connected to the same switch (S2) and (S2) has agreed that the root bridge ID is (S1) bridge ID).
We compare the lowest port priority = Equal
We compare the lowest sending port ID = Gi0/2 wins
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Since S2 (Gi0/2) wins, the link is preferred, hence, S3 (Gi0/1) will be forwarding while S3 (Go0/0) will become the blocking port.
So, guys take of yourself and have an amazing week ahead.
Bye, bye! ✌🏽