STP cost question

JeanM

If 3 switches are connected together for STP using 1Gig interfaces, and according to the revised IEEE cost chart these are cost of "4"... I get the cost of one link + cost of another link = sum of both = 8. Why is the cost between SW1 and SW3 = 5 ?

This is from Page 70 in Odom's ICND2 book.


SW1 is connected to SW2 via gi0/1 (this is cost 0+4 = 4 )
SW1 is connected to SW3 via gi0/2 (why is this cost 0 + 5 = 5 ? where is 5 coming from?)

SW2 is connected to SW1 via gi0/2 (cost here is 0+4 = 4 )
SW2 is connecred to SW3 via gi0/2 (cost here is 4+4 = 8 )

SW3 is connected to SW2 via gi0/2 (cost here 4 + 4 = 8
SW3 is connected to SW1 via gi0/1 (cost here is 0+5=5?)

Is it because the root switch/bridge advertised in BPDU that it's cost is 1? then you add 4 = 5?

Since all are Gigabit, why would that happen is what I am trying to understand.

BTW here is a good video on STP / cost - Spanning-tree, intermediate level - YouTube

chX

Unless I'm missing something, I'm wondering if it was shown as 5 for emphasis/clarity on which ports would be blocked.

JeanM

Not sure, I did check for errata and nothing on this.

JeanM

Wendell responded to me, it's simply because one interface is not using IEEE default.