200 x 301

200 x 301.2.2 Basics 41spanning-tree and show spanning-treeroot both list the root port of the local switch (assuming it is not the root switch). The exam may ask you to think about how the switch selects RPs based on the root overhead of each path to the root switch, and require some additional arbitration rules (tiebreaker), at which point the challenge becomes greater.To select its RP, the switch needs to listen for incoming hello bridge protocol data units (BPDUs). to the overhead on the incoming interface (that is, the interface on which the hello was received). The total is the root overhead for that path. The lowest root overhead will ultimately win, and the local switch will use the local port (part of the minimum root overhead path) as its root port.Although this text may be confusing, it is actually the same concept as that described in Figure 1-8 in Chapter 1 of this book.On the exam, most people will look at the problem in a slightly different way and do better if the problem comes with a LAN diagram. Instead of considering things like hello messages, they will think of the problem in terms of finding the sum of outgoing port overhead between the outgoing non-root switch and the root switch. Looking at a similar example (slightly different from the previous one), Figure 2-8 shows an example of calculating the root overhead. Note that the overhead for the GiO/3 port of SW3 has been configured to a different value.Figure 2-8 Winning SW3 Root Overhead Calculation ResultsSTP Arbitration Rules When Selecting the Root PortFigure 2-8 shows an easier process for adding the STP overhead for each outgoing interface of non-root switch SW3 to root switch SW1. It also intentionally sets up an identical situation for the purpose of discussing additional arbitration rules.When a switch selects its root port, it first selects the local port that is part of the path with the least root overhead. When these overheads are the same, the switch chooses the port adjacent to the lowest BID. This arbitration rule usually solves the problem, but not always. Therefore,To maintain integrity, the three arbitration rules, in the order in which the switches are used, are shown below.(1) Selection is based on the lowest adjacent object bridge ID. (2) Selection is based on the lowest adjacent object port priority. (3) Selection based on the lowest adjacent object internal port number. (Note that when considering these three arbitration rules, the switch will only consider the same root path.) For example, in Figure 2-8, SW3 is not the root switch, and its two paths to the root switch have an overhead of 8. The first arbitration rule