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Open Shortest Path First

aliases:

  • OSPF

OSPF stores the entire network topology graph, and chooses routes based on a configured least-cost path. While EIGRP is a flat routing system (where each router is equal), OSPF is hierarchical. Networks, their routers and their connected hosts are grouped into OSPF Areas. Routers within an area share a copy of the network topology database for the networks and hosts they serve. Routers that can connect to multiple Areas are known as area border routers. A backbone Area (referred to as Area 0) is created all the border routers, which is invisible to the routers within the other areas.

OSPF exchange "hello" messages, not only to ensure connectivity but to also ask neighbors to exchange routing information in the form of Link State Advertisement (LSA) updates. LSA updates are used to build consistent link state databases (LSDBs) that represent the network topology of the area. It then routes packets based on the shortest path first (SPF) algorithm using it's LSDB.

The smaller, more frequent updates of OSPF make it much quicker to converge and provides high performance for larger networks. It's also good for organizational purposes, since areas are used to subdivide the network into smaller sections, minimizing the amount of information needed to update each router's network topology information. However, a downside of OSPF and other link-state routing protocols is that they are a bit more resource heavy, since the overhead and calculations takes up more CPU and memory resources.

OSPF also uses it's own IP datagram format, tagged as protocol number 89. OSPF also supports either plaintext or cryptographic authentication.

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