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STP path define, STP root port define, same path issue.


How do I choose root port(RP) in spanning tree when one switch has two same paths to root switch?

Cisco file says choose the Lowest Port ID as root port.

However, I found in my practice case,

switch 2 to root switch (switch 4) has two same paths,

one is start from port 2 which port ID is 128.2

the other is start from port 3 which port IP is 128.3

according to the file, port 2 has the lowest Port IP (128.2 < 128.3).Port 2 should be the root port.

But packet tracer choose port 3 as Root Port !!!  WHY!! 


switch2's spanning tree table:


Do I understood the concept correct ?


2 Replies 2

It have one path which is FWD other is blk.

Check other side from which bpdu is receive not from this side.

The mac address via FWD path is better than BLK path (check sender of bpdu).

The lower port number rule is apply when both path come through same SW here I see two path two SW.

Martin L

It's the senders' lowest Port ID- but it may not be what made decision in your case; The process knows as tie-breaking procedure is as follows (in sequence): Once Root switch is elected (1); Next step is Port election based on the lowest Cost to Root (step 2), in case of a tie, it is lowest Bridge ID of sender (3); Then, we have the lowest Port ID of sender (4). I guess, your switch2 made choice based on step 3, lowest Bridge ID of switch 6 -assuming cost to Root is equal via 2 paths.

that being said, PT is just a simulator and its possible may have some issues; U can attach your PT file here in a zip format

Regards, ML
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