Joshua Klein

OSPF: Open Shortest Path FirstFundamentals:

• Open standard RFC 2328
• Link-State
• Builds loop-free paths that converge quickly
• Requires more processor power
• Offers flexibility in network design
• Can be more complicated

 OSPF Snapshot:

• Link-State routing protocol
• Dijkstra Shortest Path First (SPF) algorithm
• Processes more info locally to reduce network traffic
• Link-State records all possible routes which avoids looping

 Advantages over distance-vector:

• OSPF is classless and allows summarization
• Converges very quickly
• OSPF is a standard
• It conserves network bandwidth
• Uses Multicast instead of broadcast
• It sends incremental change based updates
• Uses cost as a metric

 Distance-Vector: Advertise routes to neighborsLink-State: Advertise a list of its connections.

• Change in a links state, a LSA is generated
• Neighbor is a directly connected router or a router on the other end of a WAN link with the same network address
• LSAs are shared with neighbors to create a Link-State Database (LDSB)
• LSAs are marked with a SEQ number
  • Start at 0×8000 0001 and iterate to xFFFF FFFF before wrapping around to 0×0000 0000
• Once all routers have the same LDSB, each router processes this DB with SPF and places the best routes into the routing table.
• Topology Database includes every OSPF router within the area and all the connected networks.

 Reasons routers will identify a change in the state of one of its links:

• If an LSA is new, the route is added to the database, the route is flooded out the other links so other routers are updates, and SPF is rerun.
• The router does not receive a predetermined number of consecutive OSPF hello messages.
• The router receives an LSA update from an adjacent neighbor, informing it of the change in the network topology.

In any instance these instances, the router will generate an LSA and flood it to all its neighbors with the following stipulations:

• If the LSA is new, the route is added to the db, the route is flooded out the links so other routers are updated and SPF is rerun
• If the SEQnumber is the same as the current entru in the topology table, the router ignores the advertisement
• If the LSA has a lower number than the internal DB, the router will send the newer copy from it’s DB.

 

All OSPF operations center on populating and maintaining the following tables:

·        Neighbor table

·        Topology table

·        Routing table

OSPF Hello’s include:

• RID: Router ID is a 32 bit number that is unique to this router
• Hello and dead interval: Period of time between hellos and timeout.
• Neighbor list: List of neighbor RID’s
• Area ID: Area number
• Prority: Highest priority is elected the designated router
• DR and BDR: IP address of designated routers
• Authentication: Password, if enabled
• Stub Area Flag: True if this is a stub area

After routers complete an initial exchange based on hellos, they go through a process of exchanging info about the network. After the routers have synchronized their information, they are adjacent. Once full state is reached, the tables must be kept up-to-date. LSAs are re-sent whenever there is a change, or every 30 minutes to keep them “fresh.�

 

States of OSPF:

 

·        Down: this is the first OSPF neighbor state. It means that no info has been sent or received from this neighbor.

·        Attempt: This state is only valid for manually configured neighbors in an NBMA environment. In attempt state, the router sends unicast hello packets every poll interval to the neighbor from which hellos has not been reviewed within the dead interval.

·        Init: This state indicates that the router has received a hello packet from its neighbor, but the receiving router ID was not included in the hello packet.

·        2-way: This state indicates that bi-directional communication has been established between two routers.

·        Exstart: Once the DR and BDR are elected, the actual process of exchanging link-state info can start between the routers and their DR and BDR.

·        Exchange: In the exchange state, OSPF rotuers exchange DB descriptor (DBD) packets.

·        Loading: In this state, the actual exchange of Link-state info occurs.

·        Full: In this state, routers are fully adjacent with eachother. All the router an network LSAs exchanged and the router DB are fully synchronized.