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 224.0.0.5 (all OSPF routers) or 224.0.0.6 (all Designated Routers).
• Point to Point only use multicast address 224.0.0.5
• 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. 

DR and BDR election

• 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 

     OSPF LSA Types

• 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)

Related articles

• Basic Notes on OSPF (Open Shortest Path First)
• OSPF Network Types 
• OSPF DR and BDR election
• OSPF neighbors 
• Basic notes on BGP (border gateway protocol)
• Passive Interface (RIP,OSPF and EIGRP) explained with interview questions 
• How to Prepare for NETWORK ENGINEER INTERVIEW QUESTIONS 
• Difference between Linkstate and distance vector protocols
• Notes on Administrative Distance and Metric