OSI Layer 3: Routed and Routing Protocols
65
Once adjacencies are established, the DR and BDR need to be chosen
before route information and link-state information can be exchanged. After
the DR and BDR are chosen, route information is exchanged, and the OSPF
peers continue to multicast Hello packets every 10 seconds to determine
whether neighbors are still reachable.
F I G U R E 2 . 1 6
OSPF peer initialization
Multicast Hello
packets.
Listening routers
add the new router
to the adjacency
table.
Init state
Routers reply to Hello
packet with
information contained
in Table 2.10.
Two-way state
Originating router adds all
replying routers to adjacency table.
Sub-
process
Sub-process
output
Choose DR
and BDR.
Compare all
router "priority"
values.
Is there a tie?
Yes
Compare
router
IDs.
No
Take
highest
value.
Take second-
highest value.
Exstart state
Initiate exchange
protocol.
Exchange link-state
information.
Full state
Exchange Hello
packets every 10
seconds.
Assign as DR.
Assign as BDR.
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