Layer 3 Topologies for CCNP Prep

And is there a such thing as too many routers?

When it comes to CCNP prep, what do you really need in your lab? I'm not talking about what would be nice to have, or handy, or useful once in a while. But what's enough to practice everything, or at least close to everything? Today we'll dive into the topology perspective to this question: what router topologies do you really need for the purpose of practicing? And for you experienced folks, do you think we need more? Or less? And is there such a things as too many routers in your CCNP lab?

I'll break the rules at bit and start with the conclusion: to practice most/all layer 3 features on the CCNP exam, you need 5 routers, with 4 connected to a Frame Relay cloud. You can be productive with only 3 or 4 routers, but 5 plus FR support is the sweet spot. But you're going to be tempted to go to 7 or 8 (including layer 3 switches) before you're done with the lab.

I'm breaking the topology discussion into two parts. I began thinking I'd cover it all in one day, and by the time wrote about routing, it was already a pretty healthy length for a blog post. So, today I'll look specifically at layer 3 topics in ROUTE and in TSHOOT. I'll circle back to SWITCH and the associated SWITCH topics in TSHOOT next go round.

5 routers, with at least 4 connected to Frame Relay (FR), is the sweet spot (in my opinion - in fact the whole lab series is opinion!) For the sake of discussion, let's connect all 5 to FR, which means you'll need separate hardware, or at least 6 serial interfaces in one router (one as that router's DTE interface, and 5 for the FR switch function). Physically, you'd end up with something like this:

What you get here is the ability to have multiple FR PVCs between each pair of the 5 routers. Having multiple PVCs between a pair of routers allows for layer 3 redundancy between that pair. More importantly, this one physical topology can be configured to create a logical topology that put the routers into a linear sequence (much like the sample TSHOOT topology in the TSHOOT demo), or a full mesh, or anywhere in between.  

So, why FR and not pt-pt? Most of the time, the key ROUTE and TSHOOT layer 3 learning points have no dependencies on the layer 2 technology. And when layer 2 does matter, the majority of the interesting cases use FR. When you really need the routers connected over a LAN or pt-pt, re-cable and re-config as needed.

You may have noticed that I didn't suggest that you create an MPLS cloud in the middle. MPLS is no longer a CCNP topic as an end to itself, but I could see there being topics that make you understand MPLS from an Enterprise perspective. That changes the whole idea of layer 3 adjacencies, so I could see a need to create an MPLS cloud instead of FR. For now, I'll say MPLS in a CCNP lab is overkill. But I'd love to hear opinions as to why any of you think it might be important today. (And as always, please respect Cisco's cert NDA.)

So, what could you do with this physical topology, and why do I think you need 5 routers? Some examples:

IGP redistribution: This is one of the more challenging topics on CCNP ROUTE and if the TSHOOT Demo is any indication, it's big for TSHOOT as well. You'll need 4 routers to do probably ¾ of the interesting IGP route redistribution cases, which gives you mutual redistribution between two routing domains, and a router completely internal to each routing domain.

Add a 5th router to this topology, and make that router redistribute with yet a 3rd routing domain, and you can practice with the rest of the more interesting side affects of IGP redistribution.

OSPF Multi-area: With 5 routers, you can create an OSPF design with area 0, two other areas, with 1 router internal to each area. The areas can be different stubby types, and can try to become ASBRs. For TSHOOT, I personally think it's very useful look at the OSPF database on the internal routers in particular, because you get a better sense for what's converted between areas by the ABRs. (Note that with only 4 routers, you can do the same, just sacrificing one of the internal routers.)

Matching Cisco's TSHOOT Demo Topology: You can almost match the TSHOOT Demo's string of 6 routers. In that Demo, there's an (inaccessible) ISP router, 4 traditional routers, and a layer 3 switch. If you plan on having at least 1 layer 3 switch, and get the 5 traditional routers I'm talking about today (or virtualize the routers with Dynamips), then you can re-create that exact layer 3 topo in the Demo using this 5-router FR cloud, a layer 3 switch, plus one layer 2 switch.

Practicing TSHOOT redistribution issues and redundancy issues: With 5 traditional routers, you have enough gear to create logical topologies that focus on redistribution or redundancy. The Cisco TSHOOT Demo focuses on redistribution (in my view) because the small number of routers are logicall arranged in sequence, which allows a BGP-IGP redistribution point, OSPF-EIGRP redistribution point, plus some OSPF ABRs. Alternately, you can add redundancy (multiple ABRs, for instance), or multiple redistribution points between the same two domains. Or have two routers using HSRP for redundant default gateways. While you can't build one topology to do everything at once (with 5 routers), you can keep the same cabling, change the logical topology, and practice different Layer 3 features.

So, what happens if  you're planning to use real gear, but 5 routers plus related expenses is too much $? I'd suggest getting started with what you can afford. You can practice some with 2 routers, more with 3, even more with 4. I've listed some of those tradeoffs on the CCNP topologies web page at my web site - feel free to look around there for some comments on what you can do with 2, 3, and 4 routers for CCNP prep.

And all you folks already far into CCNP, or past it, feel free to weigh in. How many routers would you recommend, and why? And just for a bit of fun, here's a poll: how many routers is truly too many for a CCNP practice lab?

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