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Open Shortest Path First
119
Exchange Routing information is exchanged using DD and LSR
packets.
Loading Link State Request packets are sent to neighbors to request any
new LSAs that were found while in the Exchange state.
Full All LSA information is synchronized among adjacent neighbors.
To gain a better understanding of how an adjacency is formed, let's con-
sider the formation of an adjacency in a broadcast multi-access environment.
Figure 4.1 displays a flow chart that depicts each step of the initialization
process. The process starts by sending out Hello packets. Every listening
router will then add the originating router to the neighbor database. The
responding routers will reply with all of their Hello information so that the
originating router can add them to its own neighbor table.
F I G U R E 4 . 1
OSPF peer initialization
Down
2Way state
Link type is
broadcast
multi-access.
Yes
No
ExStart state
Exchange
Loading
Full state
Init State
Multicast
Hello packets
Choose DR
and BDR.
Compare
Router IDs.
Take
highest value.
Take second-
highest value.
Is there a tie?
Assign as DR.
Assign as BDR.
Listening routers add
the new router to
the adjacency table.
Routers reply to Hello
packets with information
contained in Table 4.1.
Originating router adds
all replying routers
to neighbor table.
Exchange Hello packets
every 10s LSR/LSU exchanges.
(Full routing information.)
Exchange
link-state
information.
Any final LSAs
are also
exchanged.
Adjacencies must be
established (depends
on link type).
Compare all
Router Priority
values.
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