#CCNA Fast Start: Spanning Tree

certskills
By certskills July 24, 2013 11:00

Although most everyone has moved their #CCENT and #CCNA study to the new exams by now, I’m still working on a few more topics and questions for this Fast Start series. Today’s post on Spanning Tree Protocol (#STP) starts another such topic, with 4 or 5 more topics left in the series. STP is one of those topics that seem to mess with people’s heads a bit. But STP isn’t too hard – STP just takes practice before you can trust yourself with those rules!

The Exam Topics: Old and New

Cisco doesn’t tell us a lot about STP in the exam topics for the new exams as compared to the old. The following list includes all the Cisco CCNA exam topics that specifically mention STP; note that all are found in the ICND2 half of CCNA:

  • Identify Enhanced Switching Technologies
    • RSTP
    • PVST
  • Configure and Verify PVST Operation
    • Describe root bridge election
    • Spanning tree mode

First, consider the differences in the two main sections, with one including Rapid Spanning Tree Protocol (RSTP), and the other not including RSTP (and only referencing PVST). The first section mentions both RSTP and PVST, using the verb “identify”, which require more general knowledge of functions. The second section uses verbs “configure and verify”, but lists only PVST. PVST implies traditional 802.1d STP, not 802.1w RSTP.

In short, be ready to config and verify PVST mode on the switches, with 802.1d STP rules, but per the exam topics, RSTP configuration and verification isn’t in the scope of the exam. But as always, be warned – all Cisco exam topics lists are guidelines, but Cisco can include other topics.

802.1d STP and 802.1w RSTP Similarities

The most important take away from this first section of the blog, however, is that 802.1d STP and 802.1w RSTP do have important differences, the rules for choosing many important STP facts remains the same. The background messages are different, convergence is faster with RSTP (otherwise, why call it “rapid”?), and some interface roles/states differ. However, the rules for choosing the root switch, the root port, and the rules for choosing designated ports, remain the same for both 802.1d STP and 802.1w RSTP. That’s where we’ll focus in this series, and where you should focus (in my opinion) for your study.

The Rules for Choosing, and What to Choose

Per-VLAN Spanning Tree (PVST) on Cisco switches means that an STP instance (the “ST” in PVST) exists for each VLAN, or per VLAN (the “PV” in PVST). For this discussion, we’ll focus on a single VLAN at a time.

For each VLAN, the switches use STP rules to choose the following:

  1. The root switch
  2. The one root port on each non-root switch
  3. The designated port on each LAN segment

If you ignore the tiebreaker cases for steps 2 and 3 – which I will for this series – the rules are relatively simple. The tiebreakers are simple as well; they just add clutter to what I’m attempting to write about today.

Choosing the Root Switch

Choosing the root switch, for us humans, is usually pretty easy. It’s the switch whose Bridge ID (BID) is the lowest number. The only tricky part may be that some command output lists the BID in multiple parts, namely the priority (the high order part of the number) and a system ID (which is a MAC address). Sometimes, the output lists BID as three parts: the priority, the system ID extension, and the system ID, as broken down in Figure 1.

Figure 1: Format of the STP Bridge ID with System ID Extension

To find the root switch, just look at all the BIDs, and pick the lowest number. If the question lists the BIDs in their separate components, first look at the priority values, and choose the lowest priority; if a tie, choose the lowest system ID. (The system ID extension, which lists the VLAN ID, will always tie.)

Choosing the Root Port (Non-Roots Only)

Each non-root switch has to pick which ONE of its local interfaces has the least total cost path to reach the root. That is, if the local switch sent a frame to the root out each port, and you added the STP cost for all the outgoing switch interfaces in each path, what is the total cost over each path? The local switch picks the path with the lowest cost, and then picks the local interface that begins that path as its root port. The cost of that complete path called the root cost for that switch.

Choosing the Designated Ports

Finally, for any ports that are not a root port, the switch argues with any other switch attached to the same link to determine which should be the designated port on the link. The first decision criteria: the switch with the lowest root cost. To argue, each switch sends a Hello listing their root cost. The switch with the lower root cost wins, becoming the DP on that link. The switch with the higher root cost loses, and even stops sending Hellos onto the link.

Conclusions

By ignoring the tiebreakers at steps 2 and 3 for now, you can focus on the core STP logic, and get some practice. At some point, go back and read up on the tiebreaks. Next post, I’ll give you a sample question that requires that you apply the big ideas from this post using show command output as the starting point.

Answer to the #CCNA IOS Licensing Question
#CCNA Fast Start: a Spanning Tree Question
certskills
By certskills July 24, 2013 11:00
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