Interpreting show stp (1) – Answers
Of all #CCNA topics in the ICND2 half of CCNA, Spanning Tree Protocol (STP) looms with more intimidation than most. As for Mastering STP verification, the process begins with mastering the show spanning-tree command. This latest sample question focuses on the first section of that command’s output, asking about two topics: is the local switch the root switch, and what does the port number in that first grouping of messages tell you about the network? Check out the original question, and then dive in here for the answer and some explanation.
B: Some other switch is the root
E: Port G0/1 is a port on the local switch
First, note that the output shown with the question lists one section of the show spanning-tree command’s output, and does not include the 2nd and 3rd section. The first section describes facts related to the root switch, while the second section describes the local switch (that is, the switch that generated the output.) The third section shows a list of local interfaces with their STP port roles and states. So, to answer this question, you need to have mastered some of the facts found in that first section of output – you just can’t rely on the other sections (which would have been useful as well). The question requires you to trust the information in the first stanza of messages.
When issuing the show spanning-tree command on the root switch, the root switch claims to be the root, with a statement “This bridge is the root.” The output for the question does NOT list that line, so the absence of that line tells us, without question, that the local switch is not the root switch. That fact rules out two answers about which switches might be root and confirms that the local switch is not the root switch.
Figure 1: Absence of Note about the Local Switch Being Root
The second set of related answers ask about what the mention of “GigabitEthernet0/1” means. Again, from memory and earlier study, you should know that if the local switch is not the root switch, that:
- As a non-root switch, the local switch has a root port, and a cost to reach the root
- The show spanning-tree command lists those facts in the first stanza: facts about the local switch and its relationship to the root switch
As a result, you can know that the first message group’s mention of port GigabitEthernet0/1 is an interface on the local switch (SW4), namely the port used as its root port. That fact rules out two answers, and rules in the answer that restates that port G0/1 is on the local switch.
Figure 2: Location of Two Lines in First Stanza about Local Switch
As usual, let me give you a few more pointers about avoiding common mistakes on the exam. In this particular area of concern – that is, the narrow world of just the first section of output from show spanning-tree – keep an eye out for these items:
- Looking for the absence of a line of output can be one of the most challenging kinds of details to notice when under pressure on the exam. When in lab, take the time to compare that first stanza of messages on the root switch versus a non-root, and help that difference sink in visually: the claim of being the root, versus the two lines about root cost and root port.
- Another set of facts to memorize: on a non-root switch, as in this question, that first stanza lists the Bridge ID of the root, followed by the root cost and root port of the local switch. So, it’s not all about the root switch: it’s about the root switch plus the local switch’s facts related to the root switch.