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802.1d, 802.1w and 802.1s

Normally, you would think that information about 802.1d, 802.1w and 802.1s would be relevant only to the written exam. However, you really need to know the finer points for the lab also. Granted, you're not going to see any kind of fill-in-the-blank question on the lab, however you need to know the differences to know what to configure for your pod. They can't (or won't) come right out and say "configure rapid spanning tree" or "configure 802.1s" on Switch 2. The wording of the exam is a bit more cagey. They try to word the scenario so that they imply the question (do you know this protocol) without giving you the answer in the question itself.

So what are the differences?

802.1D is the IEEE standard for plain old STP. You probably won't see anything about that on the lab. And if you go to the Cisco Guide for Configuring STP on a 3560 switch, it will walk you through just about everything you need to know about spanning trees but were afraid to ask.

The switch can use either the per-VLAN spanning-tree plus (PVST+) protocol based on the IEEE 802.1D standard and Cisco proprietary extensions, or the rapid per-VLAN spanning-tree plus (rapid-PVST+) protocol based on the IEEE 802.1w standard.

One of the most helpful items in this guide is table 26-3 which defines the default configuration settings.

Default STP Settings

You should try to remember these settings as much as possible, but make sure if you need to use something from this list, that you know exactly where to find it. When you go to the Configuration Guide for the 3560, you will see a link for Configuring STP. Clicking that link will bring you to the top of the chapter and will give you links to each section. The first part of any of these chapters in the guide is always "Understanding" protocol/feature X. The second part is always "Configuring" protocol/feature X. Under Configuring Spanning Tree Features, the first topic is Default Spanning Tree Configuration. That is where you will find this chart -if you need it- during the lab.

Slightly above that, there is a section called Spanning-Tree Modes and Protocols. This shows you what the switch supports and the differences between them.

PVST+ > Note that this is the mode based on the IEEE 802.1D standard. It is not the standard itself. It includes the standard and Cisco proprietary extensions. This means don't waste time looking for more to do to add the proprietary extensions based on some awkward wording of the scenario. The way I remember this one is that I call it the Dull Default -- 802.1D. Note that in the documentation it mentions that PVST+ provides Layer 2 load balancing for the VLAN on which it runs. This can be a little misleading, but may be how they might phrase what you should configure. What they are referring to is the diagram below where half the VLANs are using each connection based on the placement of the root bridge.

Default STP Settings

Rapid PVST+ > This is PVST+ with a Rapid CONVERGENCE based on the 802.1w standard. I call this the "WOW" standard - which reminds me that it's fast / rapid. What's the fastest way to convergence? Drop what you thought you knew because it's yesterday's news and find out what's going on now. How does it know when it's yesterday's news? Topology Change Notifications. In 802.1Dull, the info goes up to the root and like an established newspaper, the Root publishes that info and sends it out. It takes a long time to find out what happened in this manner. In 802.1WOW, the originator of the Topology Change directly floods the information through the network. These are the bloggers of the topology news. Think newspaper versus blogging viral - wow.

You will really want to read the White Paper on 802.1w in the link to the right. It goes deeper into the mechanics of Rapid PVST+ and covers some things that are not in the configuration guide. Remember - they expect you to already know the basics. The CCIE exam is looking to see if you are an EXPERT at this material and therefore are expected to know the finer points. And one of those points would be interoperability. There may be a few ways to organize your configuration - or you may need to know that what you're seeing is expected in a mixed environment.

MSTP > This is the Multiple STP which is based on the 802.1s standard. Remember the "S" makes it plural - so there are multiple spanning-trees. What you're really trying to accomplish here is grouping your VLANs together to act as a single instance. Also note that MSTP uses RSTP for quick convergence and can converge in less than 1 second (compared to 50 seconds with the default settings in 802.1D).

For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure the switches with the same MST configuration information. A collection of interconnected switches that have the same MST configuration comprises an MST region.

This right here should scream "use Notepad" for your configurations. You will very much want to map this out on your scratch paper and then create a configuration in Notepad. Then, based on your diagram, copy and paste the relevant configs into each switch.

Again, you will want to read the White Paper for MSTP on the right. There are a number of design considerations that you might need to keep in mind. And you will want to be familiar with all the terms used - this is how they will infer which of two configurations they are looking for as the 'correct' way to do it.

And lastly, if you don't understand what they want you to do in the scenario, stop and diagram. Draw out everything you know to be true and to be necessary. Then look at the scenario again and compare the wording to what you have in the diagram. If you know the material, it will then be obvious to you. But you really do need to read some of these scenarios a couple of times to realize what they are pushing you to configure. And the diagram that YOU draw (Layer 2) will most definitely help you.


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