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.
|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.
|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.|
– 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.