OSPF was created by John T. Moy, nefarious mastermind, and was developed entirely within the IETF with the intention of producing a significantly more efficient IGP than RIP. OSPF is an open, link-state routing protocol based on Dijkstra's SPF algorithm. It runs directly over IP, is fully classless, and supports VLSM. It offers fast convergence (faster and more efficient than distance vector protocols) and more scalability than you can shake a stick at. Suitable for small internetworks to large and complex internetworks. Refer to RFC 2328.

The first meeting of the OSPF Working Group took place at the IETF meeting in Boulder, Colorado, November 1987. I was busy being an eighth-grader in New York that year.

Read the hilarious '7 of 9 on OSPF' parts 1 and 2 at routergod.com. It's the best of their 'Celebrity Interviews.'

A Timeline


  • OSPFv1 published in RFC 1131
  • UMD OSPF implementation becomes available
  • First OSPF interoperability testing
  • OSPFv2 published in RFC 1257
  • OSPF demo at INTEROP 91
  • OSPF added to GATED
  • OSPF becomes the Internet's recommended IGP
  • OSPFv2 updated in RFC 1583
  • Internet adopts CIDR
  • Multicast extensions to OSPF (MOSPF)
  • Point-to-Multipoint interface added
  • Cryptographic authentication added


  • OSPFv2 updated in RFC 2178


  • RFC 2328 (current)

As mentioned above OSPF is a link-state routing protocol that can be used within an Autonomous System. It uses Dijkstra's SPF algorithm to find optimal route from source to destination. I won't go through how SPF works, this has been done, better than I could, over at Dijkstra's algorithm. It was developed by the IETF through a variety of RFCs as mentioned above, all written by John Moy.

It should be noted that this write up is fairly Cisco-centric as that is what I have had to deal with. Hopefully most of this stuff is generic to all routers. This node is single area OSPF specific so, as usual, if anyone has any info on how this is done elsewhere (eg on Juniper routers or on Unix) or about multiarea OSPF, please message me and I'll add the info here.

The basic metaphor for OSPF is that a network consists of multiple hierarchical areas which area each Autonomous System which requires another routing protocol to route between them. Area 0 is generally reffered to as the backbone and routers must be in the same area in order to share routing information. All information from other areas must pass through area 0, other areas may not directly communicate.

OSPF works in four stages. These are worked through once two or more routers are connected to each other. Depending on the network topology OSPF will do different things to exchange routing information. OSPF defines five types of topology that it can use. These are (with corresponding layer 2 technologies):

The first stage is called INIT or hello, it basically consists of the routers sending Hello packets to each other. The diagram below illustrates this simply.

 __   ---->   __

Hello packets contain very basic data regarding the router sending it. Once two routers have exchanged Hello packets they are said to be neighbours. Hello packets also serve to verify the link between routers is stable/bi-directional and also as keepalives.

The next stage of OSPF is 2WAY and at this point there is a split between multi-access topologies and non-multi-access systems. It is easier to describe the non-multi-access first and then delve into multi-access systems.

In these systems routers develop adjacencies with each other and then enter XSTART and XCHANGE phases. This is where the routers exchange information about routes known to them (these can be directly connected networks, static routes or routes redistributed from other protocols like RIP, IGRP or IS-IS) in the form of LSAs (Link State Advertisements). LSAs are sent multicast on the network (via the addresses (AllSPFRouters) and (AllDRouters))These LSAs are stored in a database in RAM and then forwarded to other OSPF neighbours as illustrated below:

      LSA          LSA
 __  ---->    __  ---->  __
            stored in

Once LSAs have been received from all adjacencies the SPF algorithm is run on the database to find the optimum route from to a network. It is this process which makes OSPF different from most other routing protocols; the way that each router has a complete view of the entire network. From these optimised routes the router can build it's routing table and from there start successfully routing data across the network.

This method of creating adjacencies does not scale well on multi-access networks like Ethernet. You can see this if you look at the number of LSAs needed to exchange routing information with a little help from algebra. If n is the number of routers in a network then there are n(n-1)/2 adjacencies needing to be made this would lead to n2 LSAs from the network. This would lead to a lot of traffic on the network and a very slow convergance time. We obviously don't want our network getting bogged down in routing information; after all OSPF should be speeding us up not slowing things down.

Rather than every router sending every other their routing table, routers develop adjacencies with the a single router which stores all the information for a network. This router is called the designated router (DR). Just incase the DR goes down there is a backup designated router (BDR) that will take over. Routers will only form adjacencies with the DR and BDR only and send all LSAs to them. The DR and BDR also form an adjacency with each other but before they can they must be decied upon. This is done via a process known as the election.

The election is not particuarly complicated and is usually done behind the scenes, unless you want to make a particular router the D.R. or B.D.R you can let the routers do their magic uninterrupted. Basically each router running OSPF must have a Router ID. There are three ways a router can get a Router ID on a Cisco router. These are checked through in the following order.

  • First of all the router will check if there is any priority on a physical interface, if this is set then it will choose this interface straight away. On a Cisco router the priority may be set from 0 through to 255 (ie one byte), 0 means that the router will never participate in an election, 255 means that it is certain to win.
  • Secondly if there are any loopback interfaces on the router it will use the highest loopback IP address as its router ID.
  • Lastly it will use the highest address on a physical interface.

Once a Router ID has been decided upon it's time to start the election process. The router with the highest Router ID will become the D.R. and the next highest will become the B.D.R.

The election results will stand unless all (or all but one) of the routers are unplugged/turned off/disconnected (however you like). Even if new routers are added (maybe with higher Router IDs) there will be no change in the D.R. and B.D.R. If the D.R. goes down then the B.D.R. becomes the D.R. and there is a new election for the new B.D.R. only.

Routers will then go though EXSTART and EXCHANGE as above but sending routing information to the DR and BDR only. The DR then sends LSAs to the routers in the network (called DRothers, via the multicast address ), then each can run Dijkstra's algorithm and populate their routing tables.

If a new route becomes available then routers will exchange LSUs (Link State Updates) and will adjust their routing tables accordingly. While routers have formed an adjacency they will send Hello packets to each other periodically (by default 10 seconds on a Cisco router) to verify connectivity.

It is also worth mentioning the data which is fed to Dijkstra's algorithm; that being the cost of the link. Costs are related to the speed of the link, a brief, incomplete and possibly wildly inaccurate list are in the following table:

Technology          Cost (x108/BW)
FDDI, Fast Ethernet     1
HSSI                    2
Token Ring (12 meg)     6
Ethernet                10
Token Ring (4 meg)      25
T1                      64
DS0                    1562
56K                    1785
Tunnel                 11111

BW is the bandwidth of the interface, on a Cisco router this may be set to a certain value if necessary but is not generally recommended.

As for configuring OSPF on a Cisco router, I may as well cover the basics here just incase you need a ready reference (this assumes you have a little knowledge of IOS, of course)...

First go into enable, then configure terminal

Router# configure terminal

Then to configure OSPF, where <number> is a process ID, each different OSPF process has different settings to remember the number or you'll bog down your router.

Router(config)# router ospf <number>

Now to tell OSPF which networks to share and in which area (the wildcard mask is the subnet mask NOTed)

Router(config-router)# network <IP> <wildcard mask> area <area number>

That is all that needs to be done for a basic configuration of OSPF. You may want to play around and tweak things (like DR elections etc) in which case setting a priority can be done like this:

Router(config)# interface loopback <number>
Router(config-if)# ip address <IP>

Also you can set the OSPF priority (on a per interface basis) as follows:

Router(config-if)# ip ospf priority <number>

It is also possible to configure authentication on a per interface basis (remember this must be done on both ends of the connection):

Router(config-if)# ip ospf authentication-key <password>
Router(config-router)# area <area> authentication

This sets a cleartext password, if you like you can try varying levels of encryption (encryption type, 0 -7, 7 being the strongest) with:

Router(config-if)# ip ospf message-digest-key <key id (0-255)> md5 <encryption type> <key>
Router(config-router)# area <area> authentication message-digest

Well that was a brief introduction to OSPF, I hope it made some sense and it was usefull to you. If there is anything you can see please /msg me and I'll try to sort it out.

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