lec3

• Open Shortest Path First(OSPF)
• OSPF - An open standard, link state routing protocol that supports classless routing, VLSM/CIDR, equal cost load balancing, and authentication.
• OSPF - Each router sends out link-state advertisements(LSAs).
• OSPF - This works by using the Dijkstra algorithm, algorithm that first, a shortest path tree is constructed, and then the routing table is populated with the resulting best paths.
• OSPF - Uses only one parameter as the metric – the interface cost.
• OSPF - Uses multicast addresses 224.0.0.5 and 224.0.0.6 for routing updates.
• OSPF - Administrative distance by default is 110.
• OSPF - It provides a loop-free topology using SPF algorithm.
• OSPF - Generated Link state advertisement(LSA).
• LSA - is a data

  packet used in OSPF that contains link-state and routing information to share and learn network information.
• Link state advertisement(LSA).
• OSPF - Maintains link-state database(LDSB).
• link-state database(LDSB).
• LDSB - a collection of all LSAs received by a router.
• OSPF - It supports unlimited hop counts.
• How OSPF works

  • Form Neighbor

  • Build LSDB

  • Build Routing Table
• Neighbors - Two routers do not become neighbors unless they

  agree with the criteria
• Criteria

  Area-id - Two routers which have a common segment; their interfaces have to belong to the same area on that segment. The interfaces must belong to the same subnet

  and have a similar mask.
• Forming Neighborship

  Criteria:

  Authentication - OSPF allows for the configuration of a password for a specific area. Routers that want to

  become neighbors have to exchanges the same password on

  a particular segment.
• Forming Neighborship

  Criteria

  Hello and Dead Intervals - OSPF exchanges Hello packets on each segment. OSPF requires these intervals to be exactly the same between two neighbors.
• Forming Neighborship

  Criteria

  Stub area flag - Two routers have to also agree on the stub area flag in the Hello packets in order to become neighbors.
• OSPF Packet Types

  • Hello
  • Database Descriptor(DBD)
  • Link State Request(LSR)
  • Link State Update(LSU)
  • Link State Acknowledgement(LSack)
• Hello – Neighbor discovery and adjacency maintenance.
• Database Descriptor(DBD) – Summarizes topological database between routers during initialization of neighbor relationship.
• Link State Request(LSR) – Request Link State information from a neighbor’s database.
• Link State Update(LSU) – Advertises available links via network flooding.
• Link State Acknowledgement(LSack) – Response to flooded links sent via Link State Updates.
• Designated Router(DR) - All routing information (Link State Advertisements, LSAs) for the segment is flooded by the DR to all routers on the segment. This reduces the amount of LSAs flooding and control traffic on the segment.
• Designated Router(DR) - This reduces

  the amount of LSAs flooding and control traffic on the segment.
• Backup Designated Router(BDR) -The BDR functions as a standby for the DR. It assumes the DR role if the current DR fails for any reason.
• Backup Designated Router(BDR) - It assumes the DR role if the current DR fails for any reason
• DR AND BDR ELECTION

  RULES:

  • Routers with the highest priority will become DR and thesecond highest will become the BDR.

  • If priority is the same, then the router with the

  highest router ID is elected as DR and the router with the second-highest router ID is elected as BDR.