OSPF Basic
OSPF stands for open shortest path first and it is a Link state routing protocol .
OSPF matrix is Cost which is inversely proportional to bandwidth (108/Link bandwidth).
OSPF uses Dijikstra algorithm, also called SPF algorithm to calculate the best path for a particular destination.
OSPF use triggered update; means send update only when a change occurred in the network.
In addition it also uses Periodic updates; know as ‘link state refresh’, at long intervals such as every 30 min, also called LSA age.
During periodic update router send LSU which may contain one or more LSA, verify link is still active.
Each router independently calculates its best path to all destinations by using SPF algorithm.
OSPF runs on top of IP and uses protocol number 89.
OSPF doesn’t use any transport protocol, such as TCP, for reliability.
The protocol itself has a reliable mechanism of transportation.
OSPF is a classless routing protocol that supports variable-length subnet masking (VLSM) and discontiguous networks.
OSPF employs multicast addresses 224.0.0.5 (all SPF routers) and 224.0.0.6 (designated routers [DR] and backup designated routers [BDR]) to send Hellos and updates.