Biggest advantage of OSPF over EIGRP is that it will run on any device as its based on open standard
Each router in an OSPF network needs a unique ID that is used to provide a unique identity to the OSPF router. The router ID is chosen according to one of the two following criteria:
OSPF learns about its neighbors and builds its adjacency and topology tables by sharing LSAs OSPF routers will generate hello LSAs every 10 seconds. If a neighbor is not seen within the dead interval time, which defaults to 40 seconds, the neighbor is declared dead.
First before a router will accept any routing information from another OSPF router, they have to build an adjacency with each other on their connected interfaces. When this adjacency is built, the two routers (on the connected interfaces) are called a neighbor, which indicates a special relationship between the two. In order for two routers to become neighbors, the following must match on each router:
OSPF routers will go through three states called the exchange process:
A loop back interface is a logical, virtual interface on a router that always remains up. By default, the router doesn't have any loop back interfaces, but they can easily be created.
OSPF routers use Link State Advertisements (LSAs) to communicate with each other. One type of LSA is a hello, which is used to form neighbor relationships and as a keep-alive function. Hellos are generated every ten seconds.
When sharing link information (directly connected routes), links are sent to the DR (22.214.171.124) and the DR Disseminates this to everyone (126.96.36.199) else on the segment.
After electing the DR/BDR pair, the routers continue to generate hellos to maintain communication. This is considered an exstart state, in which the OSPF routers are ready to share link state information. The process the routers go through is called an exchange protocol
The DR and BDR form adjacencies with the other OSPF routers on the segment, and then within each adjacency, the router with the highest router ID becomes the master and starts the exchange process first (shares its link state information)—note that the DR is not necessarily the master for the exchange process. The remaining router in the adjacency will be the slave.
2. Exchange state
The master starts sharing link state information first, with the slave. These are called DBDs (database description packets), also referred to as DDPs. The DBDs contain the link-state type, the ID of the advertising router, the cost of the advertised link, and the sequence number of the link. The slave responds back with an LSACK—an acknowledgment to the DBD from the master. The slave then compares the DBD's information with its own.
3. Loading state
If the master has more up-to-date information than the slave, the slave will respond to the master's original DBD with an LSR (Link State Request). The master will then send a LSU (Link State Update) with the detailed information of the links to the slave. The slave will then incorporate this into its local link state database. Again, the slave will generate an LSACK to the master to acknowledge the fact that it received the LSU. If a slave has more up-to-date information, it will repeat the "exchange" and "loading" states.
4. Full state
Once the master and the slave are synchronized, they are considered to be in a full state. To summarize these four steps, OSPF routers share a type of LSA message in order to disclose information about available routes. Basically, an LSA update message contains a link and a state, as well as other information.
A link is the router interface on which the update was generated (a connected route).
The state is a description of this interface, including the IP address configured on it as well as the relationship this router has with its neighboring router. However, OSPF routers will not share this information with just any OSPF router.
A two-way state indicates that two OSPF routers are neighbors. A full state indicates the completion of sharing of links between routers.
Cost metric is the inverse of the accumulated bandwidth values of routers’ interfaces. The default Measurement that Cisco uses in calculating the cost metric is: cost = 108/(interface bandwidth)
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