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. 

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