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.

  3. Hi shabeer,
    I couldnt copy the content ,could you please suggest how to copy

    1. Copying is disabled.If you are looking for full PDF notes you can buy using paypal .If you are living in India ,mail me for alternative option