In this Cisco CCNA tutorial, you’ll learn about the characteristics of OSPF, and I’ll also do a comparison with our other Interior Gateway Routing Protocols of RIP and EIGRP. Scroll down for the text and video tutorials.
Cisco OSPF Characteristics Video Tutorial
Sid Wellington
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OSPF stands for Open Shortest Path First. It's a Link State routing protocol as opposed to the Distance Vector routing protocols of RIP and EIGRP. Like EIGRP, it supports large networks and has a very fast convergence time. Messages are sent using multicast rather than broadcast, so it's more efficient.
OSPF is an open standard protocol. It uses Dijkstra's Shortest Path First algorithm to determine the best path to learned networks. So, that's where it gets its Open SPF name from.
OSPF vs EIGRP vs RIP
Comparing OSPF with RIP and EIGRP, RIP has scalability limitations that we discussed in the earlier lectures, so it's not typically used in production networks. It's suitable for very small production networks or lab or test environments.
If you've got a lab and you're not testing the routing protocol, you want to test something else, so you just want to get the routing up and running quickly and easily, then RIP is a good choice for that. But not typically used in production networks because of the scalability issues, so that means your choice for your IGP comes down to either OSPF or EIGRP.
Out of the two, OSPF is the most commonly used. It supports large networks and it has always been an open standard. It is supported on all vendors' equipment, and there's plenty of support for OSPF, supported by everybody, loads of engineers understand it and are used to working on it, lots of documentation on the Internet, etc.
EIGRP, however, can be simpler to implement and troubleshoot. However, EIGRP was historically a Cisco proprietary protocol. It's an open standard now, but there's still very limited support on other vendor's equipment. Most other vendors’ equipment won't support EIGRP.
So if you wanted the simplicity of EIGRP, that meant that you were locked into using only Cisco equipment. If you wanted to be able to use any vendors' equipment, then your choice would be OSPF. That's why OSPF has been more popular than EIGRP. However, they're both very similar, they're both very good protocols.
If you want simplicity, maybe you go with EIGRP. If you don't want to have that Cisco vendor lock-in, then you can go with OSPF.
Link State Routing Protocols
OSPF is a Link State routing protocol. With our Link State routing protocols, each router describes itself and its interfaces to its directly connected neighbors. This information is then passed unchanged from one router to another.
That's the difference between Link State routing protocols and our Distance Vector routing protocols. Every router learns the full picture of the network including every router, its interfaces, and what they connect to. OSPF routers use LSA or Link State Advertisements, to pass on the routing updates.
OSPF Operations
When you enable OSPF on a router, the first thing it will do is discover its directly connected neighbors and form adjacencies with them. They will then share routes by flooding the Link State Database (LSDB). Once all of the potential routes are learned, the router will then compute the shortest path and the best routes will be installed in the routing table.
After that, the routers will respond to network changes, for example, if any new links are added, or if any links go down.
OSPF Packet Types
Different packet types are used in OSPF, the first one is the Hello packet. As soon as you enable OSPF on an interface, it will start sending out and listening for Hello packets.
When it receives a Hello packet on that interface, it will form an adjacency with the neighbor. Once the routers have formed adjacencies, they will send DBDs to each other. DBD is the Database Descriptor. That includes information about all the networks that the routers know about.
If the router is missing information about any of the networks it received in the DBD from a neighbor, it will send that neighbor an LSR, which is a Link State Request, asking for more information. The router will reply with an LSA, a Link State Advertisement.
Another packet type we have is an LSU or a Link State Update. That contains a list of LSAs which should be updated. This is used during flooding. For example, if a new link was added or if a link went down, that information needs to get flooded everywhere with an LSU.
Finally, we have the LSAck, which is the acknowledgment message. Whenever a router receives a message from a neighbor, it will send an acknowledgment back. So, this makes sure that the protocol is reliable. If a router sends out a packet and it doesn't get an acknowledgment, it will resend it.
Additional Resources
Understand Open Shortest Path First (OSPF) - Design Guide: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html
OSPF Routing Protocol Overview: https://study-ccna.com/ospf-overview/
Cisco OSPF Areas Tutorial: https://www.flackbox.com/cisco-ospf-areas
Libby Teofilo
Text by Libby Teofilo, Technical Writer at www.flackbox.com
With a mission to spread network awareness through writing, Libby consistently immerses herself into the unrelenting process of knowledge acquisition and dissemination. If not engrossed in technology, you might see her with a book in one hand and a coffee in the other.