Recent Changes in LSR

11 December 2018 | Comments Off on Recent Changes in LSR

OSPF Topology Transparent Zones

17 April 2018 | Comments Off on OSPF Topology Transparent Zones

Anyone who has worked with OSPF for any length of time has at least heard of areas—but perhaps before diving into Topology Transparent Zones (TTZs), a short review is in order. In this diagram, routers A and B are in area 0, routers C and D are Area Border Routers (ABRs), and routers E, F,…

On the ‘web: What your momma never told you about OSPF

15 August 2017 | Comments Off on On the ‘web: What your momma never told you about OSPF

Nick Russo and I talk about the OSPF routing protocol, covering some of those things you’ve probably never thought about, and giving away one of my favorite interview questions in the process. You can see the original episode at the Network Collective here.

The OSPF Two Part Metric

7 August 2017 | Comments Off on The OSPF Two Part Metric

Looking at the capabilities of any given protocol running in our networks today, it certainly seems there are few use cases left the protocol cannot support. In fact, modern interior gateway protocols have become so capable that it almost seems like we only need one to support everything. This is not reality, of course—there are…

OSPF TLVs: Taking advantage of improvements in computing power

25 January 2017 | Comments Off on OSPF TLVs: Taking advantage of improvements in computing power

OSPF was originally designed in an age when processors were much less capable, available memory was much smaller, and link bandwidths were much lower. To conserve processing power, memory, and n-the-wire bandwidth, OSPF was designed using fixed length fields (FLFs). TLVs are more difficult to process than an FLF; to process a set of FLFs,…

DR versus DIS: What’s the Diff?

10 May 2016 | 6 Comments

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…

CAP Theorem and Routing

25 March 2016 | 2 Comments

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…

Slicing and Dicing Flooding Domains (2)

9 March 2016 | Comments Off on Slicing and Dicing Flooding Domains (2)

The first post in this series is here. Finally, let’s consider the first issue, the SPF run time. First, if you’ve been keeping track of the SPF run time in several locations throughout your network (you have been, right? Right?!? This should be a regular part of your documentation!), then you’ll know when there’s a…

Slicing and Dicing Flooding Domains (1)

2 March 2016 | Comments Off on Slicing and Dicing Flooding Domains (1)

This week two different folks have asked me about when and where I would split up a flooding domain (IS-IS) or area (OSPF); I figured a question asked twice in one week is worth a blog post, so here we are… Before I start on the technical reasons, I’m going to say something that might…