CCNA Lab II: How Topologies Drive Device Choices

A Discussion of Lab Topologies, Which Determines How Many Devices to Buy

Buy one router, even an old klonky 2501, and you can get some benefit for hands-on practice for CCENT and CCNA. It's not very exciting, but you can at least try out the CLI, save configs, upgrade the IOS, and configure many features for CCNA - even if you can't test them. On the other hand, Or, you could just buy a dozen routers and a dozen switches, and you're covered for any topology that matters for CCNA. But realistically, if you want to build a CCNA lab with real gear, and your budget is not too big, the right answer is somewhere in between. Today we'll discuss how the desired topologies for the gear drive the need for a number of routers and switches, which then tells you how many of each to buy.

Context for Today's Post

This post, and this lab series, discusses topics related to building labs using real gear. We've collectively discussed tradeoffs with the other options (e.g., Simulators and Dynamips) in the past, so in this series, I'm making the assumption that we're talking about real gear.

That said, some of today's post might be helpful for those of you using Dynamips/GNS3 or a Simulator that supports building your own topologies. Why put three routers in a triangle, instead of just connecting two routers with a serial link? What can you experiment with in each case that helps you learn? These questions can only be answered well once you know the topics, but you need the gear to learn the topics… which creates a small chicken-and-egg problem.

Finally, this post is here to give us a place to discuss these issues. I keep a lot of the specific points in the topology section of my web site, but the blog is a much better place to discuss. So, ask questions about the topologies. Offer comments to help others about what worked well for you, and what didn’t. Join in!

A Buy Over Time Strategy Using Smaller Topologies

Do you want to buy all the gear, are you ok with buying a little at a time? If you want to buy all at once, you need to think a little more about what to get. Maybe get some quotes from companies that sell used gear in packages for Cisco cert study to get some perspectives. Look at the CCNA build lists on my web site. Read this series. ;-) But if you're planning to buy over time – get a few devices now, and few devices later - you might be able to get a little more comfortable with the gear before you spend your entire budget. And when you do that, you essentially need to buy just enough gear to create the smallest useful topologies for CCNA study.

For instance, you can actually do a lot with two routers and one switch. Focusing on the routers for a moment, if you get two, and have (at least) two serial interfaces on each router, then you can build a topology like this one:

What you you do with a measly two routers? How's this:

·          All admin functions (passwords, banners, IOS upgrade, change/save configs, etc)

·          IP address configuration

·          Static IP routes

·          RIP-2

·          HDLC/PPP (including CHAP)

·          ACLs

·          OSPF

·          EIGRP

·          VLSM

In fact, you can test almost everything related to routers for the ICND1 exam (CCENT Cert) using this pair of routers. in this list, everything up through the HDLC/PPP line is an ICND1 topic. (Check out this earlier RIP lab series for some more specifics).

On the downside, you cannot do everything with this small topology, and some of what you can do is pretty boring with this small topology. For instance, one of the fun things to test with routing protocols is what happens when redundancy exists. Redundancy does exist in this case, with the two parallel links, but it's pretty boring. You really need three routers to get interesting redundancy for IGPs, as in the left side of this figure, which I call the router triangle.

Small Buy for Switches: A Single Switch

If you take the same strategy with switches, planning to buy a little at a time, just buy one. (We'll get to which one later, or refer back to some related posts about CCNP from the recent past if you want to read more now.) If you buy one modern switch, say a 2950, but only one, you can test the following:

·          CLI basics

·          IP address configuration (VLAN interfaces)

·          VLAN configuration

·          Interface config (eg, auto negotiation)

·          Port security

Again, this list includes everything you need to prep for ICND1. If you also get the two routers, you can use the switch to create both LANs shown in the earlier 2-router figure, or use the routers to generate traffic for testing, as shown here:

Using a single LAN switch does present some problems when studying for the 2nd half of CCNA – in other words, the part of CCNA also included in the ICND2 exam. With one switch, you cannot test Spanning Tree Protocol (STP), Vlan Trunking Protocol (VTP), or VLAN trunking (e.g. 802.1Q). But particularly if you plan to take ICND1 first, or even study for ICND1 first before choosing the one or two exam path, then you can buy one switch, get experience, and then make a 2nd buy to complete your CCNA lab.

(For those of you who want a little more detail on the technical trade-offs between a 2-router and 3-router lab, check out this lab series from early last year:

Buying All at Once, or Completing Your CCNA Lab Topologies (Routers)

When you buy your lab all at once, or just complete the lab you starting buying a month ago, you need to consider the larger topologies that you’ll want to support. I’ll look at routers first.

You need at least three routers to test what happens with multiple competing routes in the network (see the earlier 3-router triangle.) Each router has a RIP-2 1-hop route to each of the other LAN subnets, and an alternate 2-hop route for that same subnet. But what will OSPF choose? EIGRP? And what can you configure to change those choices? You need three routers to do more meaningful experiments to test these ideas.

I personally don’t think you need a 4th router to test routing protocols for CCNA, except for one small case having to do with OSPF on LANs. OSPF routers on LANs create the concept of a Designated Router and Backup Designated Router (BDR). Each LAN that has multiple OSPF routers has one DR, and one BDR. The "others" (called DROthers) act differently towards each other. So, to have two DROthers, you need four OSPF routers on a LAN. Do you need a 4th router just to test this idea? Maybe, maybe not. You could just read about this example in Chapter 11 of my ICND2 book, assuming you have it, and not bother buying the router. 

A stronger reason to get a fourth router is to support a 3-router Frame Relay topology. You can support and test Frame Relay (FR) with three routers, as seen in the first figure below, or with four routers, as seen in the 2nd figure:

The first figure lets you configure two routers as FR DTEs, which is the configuration that is included on the CCNA exams. The third figure lets you configure three routers as FR DTEs. In each case, one router acts as a FR switch, creating the Frame Relay network.

So, do you really need three FR DTEs? To be complete, yes. FR supports more than two routers in the same subnet (layer 3), or in the same partial or full mesh (layer 2). To experiment with these configurations, you need three routers acting as FR DTEs. But can you learn a lot about FR configuration with the first figure? Absolutely.

I cannot think of a reason you need a topology that uses five routers for CCNA study, except if a lab exercise you use happens to require more devices.

A Brief Aside: One Router as FR DTE and as FR Switch

And while I hesitate to bring it up, it needs to be listed somewhere, so here we go. Those last two figures show one router acting as the FR switch, and the other 2 or 3 routers acting as FR DTEs. In reality, if you have enough physical serial interfaces, you can make one router act as both DTE and switch. How? You actually cable the router to itself.

For example, buy three routers, with one router needing 4 serial interfaces. The cabling looks like the following figure. In this example, router R1 acts like a FR switch on interfaces S0, S1, and S2, and a FR DTE on interface S3. Routers R2 and R3 are configured as a normal FR DTE.

Most people avoid this kind of setup just because of the potential for confusion, but it can save you from buying one more router – so it’s something to think about.

Completing Your CCNA Lab Topologies (Switches)

Finally, on the switch side of things, for CCNA you can consider using a pair of switches, or a triangle, as in the following figures.

With a pair, you get a big bump in what you can test:

·          VLAN Trunking

·          VTP

·          STP

However, with only two switches, the STP testing can be a little boring. You really need a third switch to get into the more interesting STP cases. I’d say the case for adding a 3rd switch is a little weaker than the case for adding a 4th router, in terms of benefit for learning. (By the way, for a little deeper comparison of STP with either 2 or 3 switches in your lab, check out this post from last year.

Final Thoughts

Ask for clarifications, make suggestions about some different ideas, or give a quick confirmation that you indeed did well with enough gear for some of these topologies.

Related Posts:

CCNA Lab Series 2011: Overview

Copyright © 2011 IDG Communications, Inc.

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