OSPF Routing Fundamentals

Link-State Routing Protocol

• Each link-state router takes a copy of the LSA, updates its link-state database, and forwards the LSA to all neighboring devices.
• LSAs cause every router within the area to recalculate routes, so it's recommended to limit the number of link-state routers in an area.
• A link is similar to an interface on a router, and the state of the link is a description of that interface and its relationship to its neighboring routers.

Why Link-State Protocol is Preferred in Larger Networks

• Link-state protocols always send updates when a topology changes.
• Periodic refresh updates are more infrequent than for distance vector protocols.
• Networks running link-state routing protocols can be segmented into area hierarchies, limiting the scope of route changes.
• Networks running link-state routing protocols support classless addressing.

Non-Backbone Internal Routers

• Non-backbone internal routers are aware of the topology within their respective areas and maintain identical link-state databases about the areas.
• The ABR router will advertise a default route to the non-backbone internal router.
• The internal router will use the default route to forward all inter-area or interdomain traffic to the ABR router.

Link-State Routing Protocol Algorithms

• Link-state routing algorithms, known as SPF protocols, maintain a complex database of the network topology.
• Each router that has exchanged LSPs constructs a topological database using all received LSPs.
• An SPF algorithm is then used to compute reachability to networked destinations.
• This information is employed to update the routing table.

Benefits of Link-State Routing Protocols

• Link-state protocols use cost metrics to choose paths through the network.
• By using triggered, flooded updates, link-state protocols can immediately report changes in the network topology to all routers in the network.
• Because each router has a complete and synchronized picture of the network, it is difficult for routing loops to occur.
• Because LSPs are sequenced and aged, routers always base their routing decisions on the latest set of information.
• With careful network design, the link-state database sizes can be minimized, leading to smaller SPF calculations and faster convergence.

Features of OSPF Protocol

• OSPF is an open standard protocol supported by many vendors.
• OSPF can run many instances in parallel.
• The metric used by OSPF is called Cost, which is equal to 100Mbps / BW in Mbps.
• OSPF update messages are triggered and incremental updates are sent as multicast messages using the addresses 224.0.0.5 & 224.0.0.6.
• OSPF periodically sends LSA refresh update messages every 30 minutes.
• OSPF update messages are called as LSU - Link State Update, which may contain many LSAs - Link State Advertisement.
• There are as many as 11 types of LSUs in OSPF.
• The Administrative Distance of OSPF protocol is 110.
• OSPF protocol supports manual summarization and VLSM.
• Route announcements carry subnet mask.