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Basic STP question

Root Bridge


| 1gb


Switch 1 - priority 2000, MAC

| PID-fa0/1 .............| PID-fa0/2

| 100mb .................| 100mb

| PID-fa0/2 .............| PID-fa0/1

Switch 2 - priority 2000, MAC –

Root Bridge is already selected; I have two switches below the root bridge.

Switch 1 is with priority set to 2000 and lower MAC address (.11) lower than switch 2.

Switch 2 is with priority set to 2000 (both switch 1 and 2 has same priority), and it has higher MAC (.22) than switch 1.

Both switches 1 and 2 has two parallel links between them, both are of same cost (100mb).

I am confused which will be the RP for switch 2 here. How will take the decision to block one of the parallel links, how will block where? Consider that cost is same for both links, but BID and port IDs are different. They have apposite Port IDs for each link.

I guess both switches will try to block their own fa0/2 because of its higher port ID, cost is same for links, but this will end up with complete blocking because higher PID interface is connected to the lower PID interface on apposite side.

Can any one explain it; this will help in making fundamental clear.


Re: Basic STP question

Hi Kapish,

When 2 switches are connected parallely like you have done cost and priority is not considered and what is left finally is the port number.

NOW port number of the switch itself is not at all considered but port id of the switch above is considered.

Your switch 1 port fa0/2 is connected to switch 2 port fa0/1 so fa0/1 on switch 2 will be blocked and port fa0/2 on switch 2 will be in forwarding state.

The reason is when BPDU travel from root switch to the lower downstream switches it will carry the port id information and once it reaches the switch 1 it will carry port fa0/2 pid to switch 2 and because it is higher so when the same BPDU reaches the last switch which is switch 2 it will compare the PID from the PID which is in BPDU which came from fa0/1 from switch 1 and because fa0/1 on switch 1 is lower so it will win and will block the fa0/1 on switch 2.



Re: Basic STP question

Thanks for this,

Still, I am confused in exact use of port cost, port priority and port ID. Which are of these changeable and the sequence of these to consider?


Re: Basic STP question

Hi Kapish,

The sequence is port cost is looked first and then the priority and finally port id.

If the 2 connection between 2 switches were with different port speed like one with gig interface and another with fast ethernet then port cost would have been the first criteria for selection and fast ethernet would have been bloked no matter what is the port id because the cost for gigabit would have been less.

But because in your setup both were fat ethernet port so cost and priority was same so the only option which was left was port id.