OSPF Notes for CCNA CCNP Preparation

shabeerm

I made this while preparing for my CCNA exams.Hope this help you guys too

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. But cisco s
· OSPF forms neighbor relationships, called adjacencies, with other routers in the same Area by
· 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.

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. : 10/40 seconds
· OSPF hello/Dead Interval time for broadcast and point-to-point interfaces. : 30/120 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

SecurityThroughObscurity

Not enough for CCNP.