Figure 10-12 RSTP Example 1

[nuseii]

Hi i'm having trouble understanding exactly how to determine the Designated Ports, Designated Bridge, and Blocked ports for this example.

In the book it's easy to pick out Switch C as the root bridge because of it's BID being the lowest. I understand that you put all of the root bridges ports into forwarding, also understood. I next understand how to find the root bridge for every other switch, it is simply the lowest cost path to the root, and taking into account link speeds(choosing two links of 4 > one link of 19 that is direct). Noting in the process that the physically shortest link doesn't always beat the most logical.

Where i'm faltering is understanding how to select a designated bridge. This isn't elaborated too much in the book and i took to online to find this out.

http://docwiki.cisco.com/wiki/Transp...liminate_Loops

"Finally, designated bridges and their designated ports are determined. A designated bridge is the bridge on each LAN that provides the minimum root path cost. A LAN's designated bridge is the only bridge allowed to forward frames to and from the LAN for which it is the designated bridge. A LAN's designated port is the port that connects it to the designated bridge."

So between Switch A and SwitchB a designated bridge must be chosen i assume, same for B and D. Note that in the book it states, "We just need a
forwarding port on the link between SwitchA and B, and since SwitchA has the lowest bridge ID, Fa0/2 on SwitchA would be a forwarding port. Any
port not listed here would go into blocking mode (nondesignated) to stop any loops."

I have read on cisco's site as well that BID is one of the many tiebreakers, AFTER the minimum root path cost. So in this case wouldn't GI0/2 on B be the Designated port since it's cost is 38(B to A, to C), as opposed to 42(A to B to D to C)?

Finally in the book it states any port not listed here will go into blocking. Would that mean that GI0/1 on Switch D would be blocking?

I hope you can answer my myriad of questions. This topic is very hard for me to grasp. Thank you for your time!