Tag Archives: ospf

Tools for TE with EIGRP

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

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). Continue reading Tools for TE with EIGRP

Five Functional Facts About OSPF

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.

Continue reading Five Functional Facts About OSPF

OSPF vs EIGRP for DMVPN

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.

Continue reading OSPF vs EIGRP for DMVPN

Packets of Interest 2012-06-12 – OSPF Refresher

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. This is a must-read.

OSPF areas and the packet types within them
http://packetlife.net/blog/2008/jun/24/ospf-area-types/

Today’s topic is a source of considerable confusion for many people new to OSPF: area types. Recall that a large OSPF domain is typically broken into separate areas to restrict the propagation of routes and reduce the amount of resources required by each router to maintain its link state database. Each area is connected to a central backbone, area zero.

OSPF relies on several types of Link State Advertisements (LSAs) to communicate link state information between neighbors. A brief review of the most applicable LSA types…

This page on cisco.com is a basic list of neighbor states. It’s short and to the point.

OSPF Neighbor States
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f0e.shtml

When OSPF adjacency is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. Those states are defined in the OSPF RFC 2328, section 10.1. The states are Down, Attempt, Init, 2-Way, Exstart, Exchange, Loading, and Full. This document describes each state in detail.

Lastly, this list of OSPF packet types is even more short and to the point. It’s perfect for turning into a flip card to memorize these facts for a cert exam.

OSPF type of packets
http://opalsoft.net/qos/OSPF-22.htm

OSPF uses Hello packets to discover and maintain neighbor relationships. Database Description (DDP) and Link State Request packets are used in the forming of adjacencies. OSPF’s reliable update mechanism is implemented by Link State Update and Link State Ack packets.