200-125 vs 200-301

200-125 vs 200-301.Cisco switches use a more obvious numbering scheme, with Fa0/l having the lowest value, followed by FaO/2, FaO/3, etc. In SW2, SW2's FaO/16 will have a lower internal port value than Fa0/17. SW1 can learn these values in Hello and will use FaO/14 as its RP.Suggestions for solving root port problems on the exam If you know where to look and some of the key commands available to lose out, you may find the RP very easy on the exam. However, the more conceptual the problem, the more you will need to calculate the root overhead on each path, relate it to the different show commands, and group the various ideas together. The following list gives some suggestions for solving STP problems on the exam. (1) If possible, find and look at the show spanning-tree and show spanning-tree root commands. They will list the root ports, and the first one will also list the root overhead (see Example 2-7). (2) The showspanning-tree command lists the overhead in two locations: the top root overhead, in the root switch section, and the bottom interface overhead, in the per-inter blood ce section. Note that the overhead at the bottom is interface overhead, not root overhead!(3) For those problems where you must calculate the root overhead of the switch, note the following.a. Remember the default overhead values: 100 for 10Mbit/s, 19 for 100Mbit/s, 4 for IGbit/s, and 2 for 10Gbit/s.b.Look for any indication of the spanning-treecost configuration command in the interface, as it will override the default overhead. Do not assume that the default overhead is already being used.c.When you know that the default overhead is already being used, also check the actual current speed if you can. The Cisco switch selects the STP overhead default based on the current speed, not the maximum speed.Determining the Designated Port for Each LAN SegmentEach LAN will have a switch that acts as the designated port (DP) for that segment. In segments where a switch is connected and the device does not use STP-for example, a segment connecting a switch to a PC or a router-the switch is always the winner because it is the only device that sends hello to the link. Nevertheless, for links with two switches, some work is needed to find2.2 Basics 43to find which is the DP. by definition: .Jin Step 1 For switches connected to the same LAN segment, the switch with the lowest overhead to reach the root, as shown in hell. sent to the link, will be the DP for that link. step 2 If there is a tie, between the individual switches with the same overhead, the switch with the lowest BID will be the DP. for example, consider Figure 2-10. the figure identifies the root, the RP, and the lowest overhead for each switch to reach the root switch via its own RP. SW2-SW4 segment: SW4 wins because it has a root overhead of 19, compared to SW2's root overhead of 20. SW2-SW3 segment: SW3 wins because it has a root overhead of 19, compared to SW2's root overhead of 20. SW3-SW4 segment: SW3 and SW4 are identical in terms of root overhead, and both have a root overhead of 19. But since SW3 has a better (lower) BID value, SW3 wins. Interestingly, SW2 does not become DP on the links to SW3 and SW4, even though SW2 has a better (lower) BID value. the DP arbitration rules use the lowest BID, but the primary DP criterion is the lowest root overhead, and SW2's root overhead happens to be higher than SW3 and SW4's root overhead.