I walked into an interesting spanning-tree issue today. I read a lot of docs about stp, but none of the articles explain the following situation:
(i have included a .vsd drawing)
Imagine you have three switches A-B-C connected in a star topology, A connects to B, B to C and C back again to A.
Link A-B = 10mbit, link B-C = 100mbit, and link C-A = 100mbit. Imagine all 3 switches have the default prio set, and A has the lowest BID, and so will become the root. Both ports on A will become designated ports.
B and C will have to discuss which ports will be root ports and which will become designated and non-designated (blocking) ports.
Because the link between A and B is 10mbps, the cost for switch B to the root is 100 (according to the new stp values). However the cost for switch B to the root via switch C will be 19+19=38. Is it the correct conclusion that switch B will block the port to switch A, the root, because of the higher cost, or are the other sides of the links from the root switch ALWAYS root ports, despite the slower link speed?
Thank you for any replies!
Yes, B will block its direct connection to the root. Do you have a live network that is not behaving that way, or is it just that you don't think this STP choice is logical?