Welcome to the SolarWinds Blog series, ‘Basics of Routing Protocols’. This is the third of a four part series where you can learn more about the fundamentals of routing protocols, types, and their everyday applications in network troubleshooting.

In the previous blog, we discussed Routing Information Protocol (RIP), its advantages, disadvantages, and how to monitor routers that use RIP in the network. In this blog, we’ll look more closely at OSPF (Open Shortest Path First) and its applications in large networks.

What is OSPF (Open Shortest Path First)?

OSPF, a link state routing protocol, is used in large organizations for their autonomous system networks. OSPF gathers link state information from available routers and determines the routing table information to forward packets based on the destination IP address. This occurs by creating a topology map for the network. Any change in the link is immediately detected and the information is forwarded to all other routers, meaning they also have the same routing table information. Unlike RIP, OSPF only multicasts routing information when there’s a change in the network. OSPF is used in complex networks that are subdivided to ease network administration and optimize traffic. It quickly calculates the shortest path if topology changes, using minimum network traffic.

OSPF allows network admins to assign cost metrics for a particular router so that some paths are given higher preference. OSPF also provides an additional level of routing protection capability and ensures that all routing protocol exchanges are authenticated.

OSPF Message Types

OSPF doesn’t send information using UDP. Instead, it builds IP datagrams directly, packaging them using protocol number 89 for the IP protocol field. Different message types of OSPF include:

  • Hello Packet – Sent by routers to set up relationships with neighbors and communicate frequently to keep the connection alive. Hello Packet shares key parameters on how OSPF is to be used within the network.
  • Database Description – The description of the link state database for autonomous systems are transmitted from one router to another.
  • Link State Request – This is requested when a portion of the network needs to be updated with current information. The message specifies exactly which links are requested by the device that wants more current information.
  • Link State Update – This contains the updated information for the requested links. It’s sent in response to the LS request.
  • Link State Acknowledgement – This acknowledges the link-state exchange process for link state update message.

OSPF – Pros and Cons

One OSPF routing protocol advantage is that it has a complete knowledge of network topology, allowing routers to calculate routes based on incoming requests. In addition, OSPF does not have limitations in hop count, converges faster than RIP, and has better load balancing. The disadvantage with OSPF is that it doesn’t scale when there are more routers added to the network. This is because it maintains multiple copies of routing information. OSPF networks with intermittent links can increase traffic every time a router sends the information. This lack of scalability means that a link state routing protocol like OSPF is not suitable for routing across the Internet.

Monitor Routers Using OSPF in Your Network

Advanced network monitoring tools have the ability to monitor network route information and provide real-time views on issues that might affect the network. By using monitoring tools in small networks, you’ll be able to view router topology, routing tables, and changes in default routes.


Learn more about other popular routing protocols like EIGRP in the blog series.