When prepend fails, what next? (1)

31 May 2016 |

So you want to load share better on your inbound ‘net links. If you look around the ‘web, it won’t take long to find a site that explains how to configure AS Path Prepending. So the next time you have downtime, you configure it up, turn everything back on, and… Well, it moved some traffic,…

What is a Failure Domain?

25 May 2016 |

“No, I wouldn’t do that, it will make the failure domain too large…” “We need to divide this failure domain up…” Okay, great—we all know we need to use failure domains, because without them our networks will be unstable, too complex, and all that stuff, right? But what, precisely, is a failure domain? It seems…

Universal Scaling and Complexity

18 May 2016 | Comments Off on Universal Scaling and Complexity

The universal scaling law is a model designed to help engineers understand transaction based systems, particularly databases and applications. What could a transaction based system have to do with network design? After all, networks aren’t really transaction based, are they? Or maybe they are… Let’s ignore the data flowing through the network for a moment…

DR versus DIS: What’s the Diff?

10 May 2016 |

OSPF and IS-IS, both link state protocols, use mechanisms that manage flooding on a broadcast link, as well as simplify the shortest path tree passing through the broadcast link. OSPF elects a Designated Router (or DR) to simplify broadcast links, and IS-IS elects a Designated Intermediate System (or DIS—a topic covered in depth in the…

Flooding Domains versus Areas

26 April 2016 |

At a fundamental level, OSPF and IS-IS are similar in operation. They both build neighbor adjacencies. They both use Dijkstra’s shortest path first (SPF) to find the shortest path to every destination in the network. They both advertise the state of each link connected to a network device. There are some differences, of course, such…

Something Old, Something New…

19 April 2016 |

Some time back a reader sent this question in— Is there some list of design fundamentals which were “true” or at least “good rules of thumb” in the past (2 months to 20 years and beyond) which are still proclaimed as true and good, which we need to throw out, or at least question closely…

The Design Mindset (5)

11 April 2016 | Comments Off on The Design Mindset (5)

So far, in our investigation of the design mindset, we’ve— Observed, specifically asking what, applying questions about state, surface, and optimization in our examination of the network as it’s actually deployed. Oriented, asking why, really focusing in on the questions around what we’re optimizing, and how that drives state and surface in the design. Decided…

The Design Mindset (4)—Interaction Surfaces

8 April 2016 |

Before talking the final point in the network design mindset, ,act, I wanted to answer an excellent question from the comments from the last post in this series: what is surface? The concept of interaction surfaces is difficult to grasp primarily because it covers such a wide array of ideas. Let me try to clarify…

The Design Mindset (3)

28 March 2016 |

So you’ve spent time asking what, observing the network as a system, and considering what has actually been done in the past. And you’ve spent time asking why, trying to figure out the purpose (or lack of purpose) behind the configuration and design choices made in the past. You’ve followed the design mindset to this…

CAP Theorem and Routing

25 March 2016 |

In 2000, Eric Brewer was observing and discussing the various characteristics of database systems. Through this work, he observed that a database generally has three characteristics— Consistency, which means the database will produce the same result for any two readers who happen to read a value at the same moment in time. Availability, which means…