OSPF Part I – Basic OSPF

Welcome to my new series focussing on technologies and expanding on it. Working from basic to thorough we will establish the skills and knowledge required to understand and configure each technology.

Open Shortest Path First is what is known as a link state protocol. Every router in an area has the exact same routing information. By using a cost based metric it is possible to determine the shortest/fastest path a destination prefix. Think of it as having two exits to a maze. One exit is 200 meters ahead but covered in long grass that would slow you down. The other exit is a paved brick path that 250 meters long. OSPF knows the paved brick path is the fastest path out of the maze. OSPF does the same with routes. It knows where a given route is, how to take the quickest of multiple paths and deliver the packet efficiently.

Let’s start with the basics.

OSPF – Open Shortest Path First

  • Transport: IP
  • Port Number: 89
  • Metric: Cumulative costs of all outgoing interfaces in a router
  • Update: / for Designated Routers
  • Updates: Full table for new neighbours, partial otherwise
  • Authentication: null, cleartext, and MD5
  • VLSM: Includes mask with each route
  • Tags: Supports route tagging
  • Summarization: Supported at Area Border Routers
  • Next-Hop: supports advertising routes with a different next-hop

OSPF uses a link state logic to calculate its routers. During the process of running OSPF between two neighbours the following occurs over three steps.

Step 1 – Neighbour Discovery involves finding the neighbouring routers running OSPF and exchanges enough information so routers know if they should become neighbours and exchange topology data. OSPF keeps a list of neighbours in the neighbour table. OSPF sends hellos to the multicast address of

Step 2 – Topology exchange requires all OSPF routers in the area to send messages to each other. Information about the topology table is stored in the topology database known as the Link State Database. Stored in the LSDB is the router-id for each routers, each routers interface, IP address, mask, subnet, and finally the list of routers reachable by each router on each interface.

Step 3 – Router computation consists of each router analyzing the top data to choose the best from its OWN perspective. The link-state algorithm used is SPF. SPF choose the shortest route for each reachable subnet/next-hop/exit interface.

Enabling OSPF can be performed under the global router process or on an interface level. Remember that on a global level that any interface that has an IP address that is covered by a network range statement in OSPF will be included in the routing process. OSPF will check for additional information when forming adjacencies with neighbours.

The following information is sent in a hello. Hello packets are a critical part of the process of establishing and maintaining OSPF neighbours.

Stub Area flag, Router Priority,
Dead Interval, Hello Interval,
DR IP, BDR IP, Subnet Mask,
Authentication Digest,
List of Neighbours.

Alright – The first part is done for now. Tune into part II shortly for more information regarding Designated and Backup Designated Routers, Router ID’s and an introduction to LSA types.



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