CCO contains a wealth of information for those preparing for the CCIE exam. Unfortunately, that same information is not necessarily good preparation for the CCDE exam. The CCDE won’t be testing your CLI skills, or even your Cisco feature implementation skills. No, the CCDE claims to test timeless network design skills, which requires something different. Design is considered a dark art, since most of the answers begin with ‘it depends’. The trouble is, you’re expected to know what the conditions for deployment will be, and as network designers we work with imperfect information. Customer requirements are fuzzy since customers sometimes don’t know what they want. Feature parity between platforms can be frustrating. The CCDE will most likely be testing your ability to see big-picture issues instead of digging into the weeds. Nobody but the CCDE creators know, of course, but we can infer this information by the reading list and written outline.
Let’s look at the first topic: IP routing
- Explain route aggregation concepts and techniques.
- Purpose of route aggregation
- Scalability and fault isolation
- How to Aggregate
Nothing in here will be specific to Cisco products. Nothing in here will be feature/platform dependent. What is in here, however, is IP routing protocol dependent.
What is the purpose of route aggregation?
We aggregate to scale our networks. By aggregating, we reduce the impact of a link flap in one area of the network. We hide topology through aggregation. Link-state and Distance-vector routing protocols aggregate in different ways. If we want to hide topology in link-state, we need to use areas for aggregation. Distance-vector by definition hides topology.
Do you notice what’s left unsaid in the above question? I think a key question you need to ask yourself is:
What are the problems with route aggregation?
Route aggregation hides topology. Route aggregation can result in suboptimal routing. How do you fix these issues if you need to? Why should you care? Topological information hiding can be useful, but can be detrimental in the case of Traffic Engineering and Mobility. With Mobility, as your users move around the network you will be unable to aggregate as effectively. Suboptimal routing is something you may not care about immediately, but it could cause problems for your customers depending on link speed, etc, and you should know how to address these issues in your design.