In response to my article about what would cause a directly connected route to be overridden, Matt Love (@showflogi) made a good observation:

Good stuff - LPM rule can be a useful tool if you want to manipulate paths without mucking with metrics, esp if using multiple protocols

— Matt Love (@thatmattlove) July 13, 2017

What Matt is saying is that longest prefix match (LPM) is a mechanism that can be used to steer traffic around the network in order to meet a technical or business need. This type of traffic steering is called traffic engineering (TE).

It's funny, in my experience, OSPF is the most widely used interior gateway protocol because it "just works" and it's an IETF standard which means it inter-ops between different vendors and platforms. However, if you really start to look at how OSPF works, you realize it's actually a highly complex protocol. So on the one hand you get a protocol that likely works across your whole environment, regardless of vendor/platform, but on the other you're implementing a lot of complexity in your control plane which may not be intuitive to troubleshoot.

This post isn't a judgement about OSPF or link-state protocols in general. Instead it will detail five functional aspects of OSPF in order to reveal-at least in part-how this protocol works, and indirectly, some of the complexity lying under the hood.

In this post I'm going to look at the characteristics of OSPF and EIGRP when used in a Dynamic Multipoint VPN (DMVPN). I will do my best not to play favorites and instead stick to the facts (yes, I do have a preference :-). To that end I will back everything up with data from my lab. The focus areas of the comparison will be:

• Scalability of the hub router's control plane
• Overall control plane stability
• Traffic engineering

This post won't go into any background on how DMVPN works. If you're not yet familiar with DMVPN, I recommend watching these introductory videos by Brian McGahan. This post also does not do a deep dive on OSPF or EIGRP. I'm making the assumption that you're already familiar with the different LSA types in OSPF and general functions of EIGRP.

After reading this post you should be able to describe the pros and cons of OSPF and EIGRP in the three areas listed above and incorporate this knowlege into a DMVPN design.

I was recently brushing up and refreshing my OSPF knowledge and I discovered some great resources that I wanted to document for my future use and also share with others. I found these resources great for explaining/detailing area types, packet types, and neighbor states. This first document hosted at packetlife.net is one of the best explanations of packet types I've ever seen due to the visual aids that the author, Jeremy Stretch, incorporated.