Understand Open Shortest Path First (ospf) Design Guide

Exstart: Routers try to establish the initial sequence number to be used in the information
exchange packets. The sequence number insures that routers always get the most recent
information. One router becomes the primary and the other becomes secondary. The primary
router polls the secondary for information.
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Exchange: Routers describe their entire link-state database through sent database
description packets. At this state, packets could be flooded to other interfaces on the router.
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Load: At this state, routers finalize the information exchange. Routers have built a link-state
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request list and a link-state retransmission list. Any information that looks incomplete or
outdated are put on the request list. Updates are put on the retransmission list until
acknowledged.
Full: At this state, the adjacency is complete. The neighbor routers are fully adjacent.
Adjacent routers have a similar link-state database.
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Here is an example:
RTA, RTB, RTD, and RTF share a common segment (E0) in area 0.0.0.0. These are the
configurations of RTA and RTF. RTB and RTD must have a similar configuration to RTF and are
not included.
RTA#
hostname RTA
interface Loopback0
ip address 203.0.113.41 255.255.255.0
interface Ethernet0
ip address 203.0.113.141 255.255.255.0
router ospf 10
network 203.0.113.41 0.0.0.0 area 1
network 203.0.113.100 0.0.255.255 area 0.0.0.0
RTF#
hostname RTF
interface Ethernet0
ip address 203.0.113.142 255.255.255.0
router ospf 10
network 203.0.113.100 0.0.255.255 area 0.0.0.0

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This is a simple example that demonstrates a couple of commands that are very useful in
debugging OSPF networks.