Introduction to OSPF PDF

Introduction to OSPF  Arouting algorithm within an AS is called an Interior Gateway Protocol, and an algorithm for routing between ASes is called an Exterior Gateway Protocol.  The original Internet interior gateway protocol was a distance vector algorithm (DVA) based on Bellman-Ford...

Introduction to OSPF  Arouting algorithm within an AS is called an Interior Gateway Protocol, and an algorithm for routing between ASes is called an Exterior Gateway Protocol.  The original Internet interior gateway protocol was a distance vector algorithm (DVA) based on Bellman-Ford algorithm. However, it was replaced by a link state protocol in May 1979. In 1990, the successor, called OSPF (Open Shortest Path First) became a standard  Characteristics of OSPF: 1. Open. 2. A variety of metrics is supported, including physical distance, delay, and so on. 3. Dynamic. 4. Load balancing (Splitting the load over multiple lines). 5. Hierarchical systems are supported 6. Security is provided (Preventing from spoofing router by sending them false routing information). 7. Has two types OSFV2---support IVP4 and OSPFV3---- support IPV6 8. OSPF supported updated automatically without confitrmed from another routers  Distance metrices 110  Cost Metric: OSPF uses a cost metric based on link bandwidth to calculate the best path. A higher bandwidth link has a lower cost, and OSPF will prefer the path with the lowest total cos  OSPF achieves rapid convergence : by propagating link-state advertisements (LSAs) throughout the network. When a network topology change occurs, OSPF routers quickly recalibrate their routing tables to reflect the new topology.  Used LSAs (Link-State Advertisements) OSPF routers send LSAs to advertise the state of their links to neighbors. These LSAs are stored in the link-state database, which all routers in the same area share.  Used Dijkstra’s Shortest Path First (SPF) Algorithm  OSPF uses Dijkstra's SPF algorithm to compute the shortest path to each destination based on the link-state database.  OSPF sends routing updates using multicast addresses (224.0.0.5 for all OSPF routers and 224.0.0.6 for designated routers), reducing network traffic compared to protocols that use broadcasts.  OSPF supports multiple authentication methods (including clear-text and cryptographic MD5 authentication) to secure routing exchanges between routers.  Support VLSM OSPF Network Types  OSPF Areas  Area is group of link and routers that share the same LSDB Single-area OSPF: one area with same LSDB. Multiarea : each Multiple area have separate LSDBs.  Link state Advertisement (LSA) and link state database (LSDB) LSA: OSPF message contains routing information and reflooded every 30 minute by default. However, when LSA change it will be flooded immediately. LSDB is database contains of LSA , and is located in each router  Router Types Internal Routers: Routers located entirely within one area. Backbone Routers: Routers in Area 0 (the backbone area). Area Border Routers (ABRs): Routers connecting multiple areas. Autonomous System Boundary Routers (ASBRs): Routers connecting to external networks, such as other autonomous systems or BGP networks. Area 1  Neighborship It is exchanging information between routers (Hello message) Hello message is updated by using Multicast Ip address 244.0.0.5 Hello message consist of from : 1. Hello message 2. Area Id 3. Network area 4. Authentication 5. Neighbors HANAN  OSPF States: OSPF neighbor relationships progress through the seven states, one at a time in the following order: Down Init Two-way ExStart Exchange Loading Full Adjacency  Down State: In the Down state, the OSPF process has not yet exchange information with any neighbor. OSPF is waiting to enter the Init State. Init State: OSPF routers send Type 1 (Hello) packets at regular interval (usually 10 seconds) to establish relationship with neighbor routers. When an interface receives its first Hello packet, the router enters the Init state, which means the router knows a neighbor is out there and is waiting to take the relationship to the next step.  Two-Way State: Hello packets include a list of the sender’s known OSPF neighbors. A router enters the two-way state when it sees itself in a neighbor’s Hello. The routing information is not shared between routers in two-way state. To learn about other routers’ link state, every OSPF router must form at least one adjacency  ExStart State: The two neighbor routers use Hello packets to negotiate who is the master and who is the slave in their relationship. The router with the highest OSPF router ID becomes the master. When the neighbors establish their roles as master and slave, they enter the Exchange state and begin sending routing information.  Exchange State: Neighbor routers use Type 2 database description (DBD) packets to send each other their link- state information. The routers compare what they learn with their existing link-state databases. If either router of the router receives information about a link that is not already in its database, the router requests a complete update from its neighbor.  Loading State: After each router describes its database to the others, the router can request information that is more complete by using Type 3 packet, Link State Requests (LSRs). When a router receives an LSR, it responds with an update using Type 4 Link State Update (LSU) packet. These Type 4 LSU contain the actual linkstate advertisements (LSAs), which is the heart of the link- state routing protocols. Type 4 LSUs are acknowledged using Type 5 packets, called Link-State Acknowledgements (LSAcks).  Full Adjacency: With Loading state complete, the routers are fully adjacent. Each router keeps a list of adjacent neighbors, called the adjacency database. Example  Steps in OSPF Operations: OSPF routers progress through five distinct steps operation: 1. Establish router adjacencies. 2. Elect a DR and BDR (if necessary). 3. Discover routes. 4. Select the appropriate routes to use. 5. Maintain routing information.