Brief Note on OSPF | Network Educator

Brief Note on OSPF

  • OSPF stands for Open Shortest Path First.
  • It’s 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 algo.
  • 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 (all SPF routers) and (designated routers [DR] and backup designated routers [BDR]) to send Hellos and updates.


How OSPF works when changes occurs in Network ?

As we know OSPF uses Triggered updates, as any network changes being observed by router it generate the triggered in form of LSA which have information about concern link & propagates to all neighbor device with a special multicast address. Each routing device take a copy of LSA, updates its link-state database (LSDB) & fwd the LSA to its entire neighbor. The purpose of flooding LSA is to ensure that all routing device updates their database before updating routing tables to reflect new topology. LSDB is used to calculate the best path through the network. SPF algorithm run against LSDB & form SPF tree. The best paths are then selected from the SPF tree and placed in the routing table.