Topology Design for Routers Through the Lens of a Lab Exercise

An Advanced CCNA Lab, a Challenge for CCNP, and Some Discussion of Lab Topologies

Do you need two routers to practice routing commands for CCNA? Or do you really need 3 - or even 4? Likewise for CCNP, or one of the CCNA concentrations? And what topology do you need to use - which determines what interfaces to buy - so that you can test every function of every config option for that exam, and see the nuances that you might miss in a different topology? And what are those nuances in the first place? And what lab exercises should I use to learn all this? Well, you asked for it, and I liked the idea, so I'll blog it: some lab exercises that at the same time give us a backdrop to discuss lab topologies.

By the way - thanks to all of you who gave me a ton of ideas for the blog in coming months. I'll be referring back to that blog and your suggestions throughout the year. This post is the first one that's in reaction to some of your suggestions - thanks!

First: Topology. When labbing at home with your own gear or with Dynamips, you have some topology choices to make. Those choices imply the number of routers, the number of interfaces, as well as issues of IOS version, feature set, and features supported on different router families. I'm ignoring the IOS version related stuff for this post, focusing on number of routers and number/types of interfaces. My goal is to use lab exercises to let us discover together why a topology worked well for parts of a lab, and why it posed problems for other parts of a lab. This exercise should be doable on any IOS feature set, at any version from 12.2 and beyond, and probably even older images than that.

Regardless of whether you're a CCNA yet or not, it can be useful to take 2 minutes and type your ideas for configs, just for the reason of thinking about the topology. After thinking about the config, ask yourself questions about the topology.

  1. What problems did you see?
  2. What made you suspicious?
  3. Do I need a 3rd router, with a triangle of routers for the topology?
  4. Or do you not really need a 3rd one, but it would be easier to think about this problem if it were 3 routers?
  5. Which part of the lab - which commands - might be more easily learned, or better learned, with a different topology - and which topology?

I will address some of these questions in the context of the lab next time around.

A quick aside: For those of you who are wondering, work priorities took over, and my Dynamips Journey is on temporary hold for a few weeks. Short version: Looks like a Core 2 Duo with 4G RAM is the least CPU/memory for which the PC, running Linux, would have the power to run 12 router images, which was my original criteria. I intend to get back to it, but I'd rather not blog about it until I've gotten through more of the work.

Next: the Lab. The lab I'm suggesting today is a CCNA level lab, but it does require configuration skills on the high end of CCNA. However, every command used in the lab is also in my CCNA books, so we'll call it a CCNA level lab. I plan to do another such post in the near term with another lab, and again talk through the topology, but with a CCNP level EIGRP lab.

So, the topology (see figure below) has 2 routers,  with 2 serial and 1 LAN interface per router. Why? Well, that's easy and cheap to buy. You could actually do this with a pair of 2501 routers, a pair of serial back-to-back cables, and spend maybe $50-60 bucks (without Ethernet transceivers). See my note at the end about doing this lab on 2501's, which require external transceivers, without spending the $. Or a pair of 2610's with 2 serial back-to-back cables also for not a lot of cash. While some labs might require a FastE rather than Ethernet interface, this particular lab does not.

The lab, at least for our purposes, is a set of criteria, as follows:

1)    Configure IP addressing per the list below, and bring up interfaces, such that each router can ping it's own interface IP addresses.

2)    Confirm that each router can ping the other router's serial IP addresses.

3)    Configure RIP such that R1 has exactly one route for subnet, which uses R1's S0/0/0 (or equivalent in your pod) as the outgoing interface.

4)    Configure and test so that when you shutdown R1's S0/0/0 (or your equivalent) is shut down, R1 learns and uses ( as seen in show ip route) the alternate route for through R1's S0/0/1 interface.

5)    Use no unnecessary configuration commands.

6)    Avoid commands that are clearly outside the scope of CCNA.

Subnet requirements:

  • R1's LAN: subnet Give R1 the lowest IP address in the subnet.
  • R2's LAN: subnet Give R2 the highest IP address in the subnet.
  • Top serial link: subnet Give R1 the lower IP address, and R2 the higher address.
  • Bottom serial link: subnet Give R1 the lower IP address, and R2 the higher address.

CCNA Candidates: If you're using this lab to learn the commands, I'd suggest avoiding reading all comments added here until you've done the lab. If for some reason I need to clarify something about the lab, I'll edit this post, rather than adding a comment.

CCNP/CCIE Candidate:  If you want a challenge, if you remove the "use only CCNA level commands" restriction, there's an alternative solution that might be interesting to work through. I'll list it next week as well.

SPOILER ALERT: Final thing, if you post about a possible solution, please don't put the key part of that solution in the title of your post, just to avoid spoiling it for others.


PS To get a router's LAN interface to reach up/up state after being "no shutdown", even with no cable connected or an external transceiver on a 2500 series router, just add the "no keepalive" command to the interface.

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