OSPF Theory

#ospf #ospf #ospf #ospf #details #theory #ospf LSA #ospf  #packet type #ospf states #ospf states

All of the following fields in an OSPF hello message must match for an adjacency to form:
-hello timer
-dead timer
-area ID
-authentication type
-stub area flag

Point-point interfaces: hellos every 10 seconds, 40 second dead timer .
Non-broadcast multi-access (NBMA) interfaces: hellos every 30 seconds,120 second dead timer

6-Packet Types:
Hello-Used to establish communication with dir ectly connected neighbors
Database Descriptor (DBD) – Lists router IDs from which the router has an LSA and its current sequence number
Link State Request (LSR) – Request for an LSA
Link State Update (LSU) – Reply to an LSR with ther equested information
Link State Acknowledgment (LSAck) – Used to confirm r eceipt of link-state information

7-OSPF States:
There are 7 different OSPF states when forming neighbor relationships. Take
the time to learn the states and their corresponding functions.
1. Down State
OSPF has not started and no hellos have been sent.
2. Init State
Hellos ar e sent out all OSPF-participating interfaces.
3. Two-way State
A hello is r eceived fr om another r outer with its own RID in the neighbor field.
All other required elements match and the routers become neighbors.
4. Exstart State
Routers determine which one will begin the route exchange process with the other .
5. Exchange State
Routers exchange DBDs.
6. Loading State
Routers compare the DBD to their LS database. LSRs are sent out for missing or outdated LSAs. Each router then responds to the LSRs with a Link State
Update. Finally , the LSUs are acknowledged.
7. Full State
The LSDB is completely synchr onized with the OSPF neighbour . The Open Shortest Path First (OSPF) All OSPF Routers address is used to send Hello packets to all OSPF routers on a network segment. The OSPF All Designated Routers “”(DR)”” address is used to send OSPF routing information to designated routers on a network segment.

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