Basic Notes on OSPF (Open Shortest Path First)

OSPF (Open Shortest Path First)

OSPF is a standardized Link-State routing protocol, designed to scale efficiently to support larger networks.

OSPF Characteristics
  • OSPF employs a hierarchical network design using Areas.
  • OSPF will form neighbor relationships with adjacent routers in the  same Area.
  • Instead of advertising the distance to connected networks, OSPF  advertises the status of directly connected links using Link-State  Advertisements (LSAs).
  • OSPF sends triggered updates only and send only changes
  • LSAs are additionally refreshed every 30 minutes.
  • OSPF traffic is multicast either to address (all OSPF routers) or (all Designated Routers).
  • Point to Point only use multicast address
  • OSPF uses the Dijkstra Shortest Path First algorithm to determine the shortest path.
  • OSPF is a classless protocol, and thus supports VLSMs.
  • OSPF supports only IP routing.
  • OSPF routes have an administrative distance is 110.
  • OSPF uses cost as its metric, which is computed based on the bandwidth of the link.
  • OSPF COST = Reference bandwidth/Link Bandwidth
  • OSPF has no hop-count limit. 
  • OSPF forms neighbor relationships, called adjacencies, with other routers in the same Area.
  • All routers must be connected to area 0 (Backbone Area)
  • All Routers in an AREA have same topology table
  • OSPF summarice networks in ABR (Area Border Router)
  • One Area contain localized updates.
  • ASBR (Autonomous system Border Router) Connects OSPF with other dynamic protocols like EIGRP OR RIP
  • only ABR and ASBR can summarize in OSPF
  • OSPF only become neigbour with routers in same area
  • In every single network in OSPF having a shared segment,there will be a DR  and BDR
  • In a shared ethernet segment ,Only DR and BDR will be in FULL state and others might be in 2 Way state 
The OSPF process builds and maintains three separate tables:
  • A neighbor table – contains a list of all neighboring routers.
  • A topology table – contains a list of all possible routes to all known  networks within an area.
  • A routing table – contains the best route for each known network. 
Different types of routers in OSPF
  • Routers in the backbone area (area 0) are called backbone routers
  • Routers between 2 areas (like the one between area 0 and area 1) are called area border routers (ABR) 
  • Routers that run OSPF and are connected to another network that runs another routing protocol (for example RIP) are called autonomous system border routers ( ASBR) 
Each OSPF router is identified by a unique Router ID. The Router ID can be determined in one of three ways:
  • The Router ID can be manually specified.
  • If not manually specified, the highest IP address configured on any Loopback interface on the router will become the Router ID.
  • If no loopback interface exists, the highest IP address configured on any Physical interface will become the Router ID 
Hello / Dead Interval
  • OSPF hello/Dead Interval time for non-broadcast and point-to-multipoint interfaces.  : 30/120 seconds
  • OSPF hello/Dead Interval time for broadcast and point-to-point interfaces.  : 10/40  seconds
  • Notice that, by default, the dead interval timer is four times the Hello interval. 
  • First they look at Router Priority. BY Default Router priority is One.We can change it if we need
  • If Router priority is same,OSPF will look at Highest Router -ID for DR BDR election
  • If we set Router priority is O,that router will not participate in DR/BDR election
  • In FrameRelay (NBMA -non broadcast multi access) network ,HUB Must be elected as DR .We can do this by changing router priority 
  • Type 1 - Represents a router
  • Type 2 - Represents the pseudonode (designated router) for a multiaccess link
  • Type 3 - A network link summary (internal route)
  • Type 4 - Represents an ASBR
  • Type 5 - A route external to the OSPF domain
  • Type 7 - Used in stub areas in place of a type 5 LSA
   OSPF Area types
  • Backbone area (area 0)
  • Standard area
  • Stub area
  • Totally stubby area
  • Not-so-stubby area (NSSA)
  OSPF Network Types
  • Point-to-Point
  • Point-to-Multipoint
  • Broadcast
  • Non-Broadcast
  • Non-Broadcast Multi-Access (NBMA)
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  1. Hello timers I guess should be reversed. broadcast 10/40 and Non broadcast 30/120 sec.

  2. The hello and dead timer for Broadcast and P2P network is 10sec and 40sec respectively and for the NBMA network it is 30sec and 120sec.