Since my new job requires me to be good at OSPF and the fact that the last time I studied OSPF was during my CCNA studies 4 years back I had to come up with this so I can fetch it when needed.
Just some notes that I prepared from CBT nuggets. Mostly it’s what Jeremy had on his screen and some of it is from the ROUTE cert guide. Just collated them for it to be in a single place for reference.
The tables
| Neighbor table | Stores the list of OSPF neighbors |
| Topology table | A.k.a LSDB, it represents the entire diagram of the network (Area) – This table is built using the following; – Router ID of all the routers in the area – Router interface, IP address, mask and subnet of those routers – List of the routers reachable by each router on each interface. |
| Routing table | This table keeps the best path to reach each of the prefix/subnet based on the SPF algorithm performed on the topology table (LSDB). |
OSPF Area Design consideration
- All areas must connect to Area 0 (backbone area).
- All routers in an Area have the same topology table.
- The goal is to localize updates within an area.
- Requires hierarchical design in terms of IP addressing. Use summarization and avoid using discontiguous networks within an area.
Basic OSPF Terms
| LSDB | Link State Database This stores the topology data. |
| SPF | Shortest Path First Algorithm that analyzes the LSDB to determine the best path for each prefix i.e. to calculate the routing table once the neighborship is formed. |
| LSU | Link State Updates OSPF packet that contains LSAs. |
| LSA | Link State Advertisement Holds topology information. Held in memory in the LSDB and propagated over the network in LSU messages. |
Router types
| ABR | Area Border Router – Has interfaces connected to at least two different OSPF areas including the backbone area (area 0). – Holds topology data for each area and calculates routes for each area. – Advertise only routes between each area. Does not share topology data between areas. |
| Backbone router | Router that has at least one interface connected to the backbone area. |
| Internal router | A router that has interfaces connected to only one area. |
| DR | Designated Router – On multi-access data links like LAN, a DR is elected by the OSPF routers on that data link to perform some special functions in regards to OSPF. |
| BDR | Backup Designated Router – On a multi-access data link, the BDR monitors the DR and becomes prepared to take over the DR, should the DR fail. |
