Open Shortest Path First ( OSPF ) notes

The following requirements must match to become an OSPF neighbor

  • subnet mask used on the subnet
  • subnet number
  • hello interval
  • dead interval
  • ospf area ID
  • authentication checks ( if in use )
  • value of the stub area flag
OSPF Database Exchange Process
  • based on ospf interface type, the router may or may not collectively elect a DR designated router and backup designated rotuer BDR
  • for each pair of routers that need to become fully adjacent, mutually exchange the contents of their LSDBs
  • When completed the neighbors monitor for changes and periodically reflood LSAs while in Full the adjacent state.
Designated Router Election Process notes

  • Router sending the Hello with the highest OSPF priority setting becomes the DR
  • if two or more routers tie with the highest priority then the router with the highest RID wins
  • typically the router with the second-highest priority becomes BDR
  • priority value of 0 means the router will not partake in the election
  • priority values 1 - 255 are considered in the election
  • If a new router comes online after the election, the election is not recalculated.
OSPF Neighbor States


OSPF design terminology

  • Area Border Router ABR - Anospf router with interfaces connected to the backbone area and to at least one other area
  • Autonomous System Border Router ( ASBR ) - connects to routers that do not use ospf for the purpose of exchanging external routers into and out of the opsf domain
  • Backbone router - a router in one area, the backbone area
  • Internal router - a router in a single non backbone area
  • area - a set of routers and links that share the same detailed LSDB information
  • backbone area - a special ospf area to which all other areas must connect - area 0
  • external route - a route learned from outside the ospf domain and then advertised into the ospf domain
  • intra-area route - a route to a subnet inside the same area as the router
  • interarea route - a route to a subnet in an area of which the routers not a part
  • autonomous system - a reference to a set of routers that use ospf