Very basic OSPF stuff

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

  1. All areas must connect to Area 0 (backbone area).
  2. All routers in an Area have the same topology table.
  3. The goal is to localize updates within an area.
  4. 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.


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