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ROUTING

EXCELLENT

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GUESTBOOK

10. Explain how OSPF discovers, chooses, and maintains routes

Steps of OSPF Operation

1. Establish router adjacencies
2. Elect a designated router and a backup designated router
3. Discover routes
4. Select appropriate routes to use
5. Maintain routing information

When OSPF adjacency is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. Those states are defined in the OSPF RFC 2328, section 10.1. Following is an explanation of each state.

1. Down
   This is the first OSPF neighbor state. It means that no information has been received from this neighbor, but Hello packets can still be sent to the neighbor in this state. If a router doesn't receive a hello packet from a neighbor within the RouterDeadInterval time (RouterDeadInterval = 4*HelloInterval by default), then the neighbor state changes from Full to Down.
2. Attempt
   This state is only valid for neighbors in an NBMA environment. Attempt means that the router is sending hello packets to the neighbor, but has not yet received any information.
3. Init
   This state specifies that the router has received a hello packet from its neighbor, but the receiving router's ID wasn't included in the hello packet. When a router receives a hello packet from a neighbor, it should list the sender's router ID in its hello packet as an acknowledgment that it received a valid packet.
4. 2-Way
   This state designates that bi-directional communication has been established between two routers. Bi-directional means that each router has seen the other's hello packet. At this state, a router decides whether to become adjacent with this neighbor. On broadcast media, a router becomes full only with the designated router (DR) and the backup designated router (BDR); it stays in the 2-way state with all other neighbors.
5. Exstart
   This is the first state in forming adjacency. It is used to elect the master and slave, and to choose the initial sequence number for adjacency formation. The router with the higher router ID becomes the master, and as such, is the only router that can increment the sequence number.
6. Exchange
   In the exchange state, OSPF routers exchange link-state advertisement (LSA) information. Each database description (DBD) packet includes LSA entries from the OSPF master router's link-state database and has a sequence number that is explicitly acknowledged. Routers also send link-state request packets and link-state update packets (which contain the entire LSA) in this state.
7. Loading
   In the loading state, routers send link-state request packets. During the adjacency, if a router receives an outdated or missing LSA, it requests that LSA by sending a link-state request packet.
8. Full
   In this state, routers are fully adjacent with each other. All the router and network LSAs are exchanged and the routers' databases are fully synchronized.
   Full is the normal state for an OSPF router. If a router is stuck in another state, it's an indication that there are problems in forming adjacencies. The only exception to this is the 2-way state, which is normal in a broadcast network. Routers achieve the full state with their DR and BDR only. Neighbors always see each other as 2-way.

OSPF will send summary-update if it doesn't received any updates within 30 minutes.

OSPF maintains these databases

1. Adjacencies database
2. Routing table
3. Topology database

These parameters mush match to form adjacencies

1. Area ID
2. Authentication type and password
3. Hello and dead intervals
4. Stub area flag if available