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When an OSPF router has installed routes in its
routing table, it must diligently maintain routing information. When there is a
change in a link-state, OSPF routers use a flooding process to notify other
routers on the network about the change. The Hello protocol's dead interval
provides a simple mechanism for declaring a link partner down. If RTB does not
hear from RTA for a time period exceeding the dead interval (usually 40
seconds), RTB declares its link to RTA down.
RTB then sends an LSU packet containing the new
link-state information, but to whom?
- On a point-to-point network, no DR or BDR
exists. New link-state information is sent to the 224.0.0.5 multicast
address. All OSPF routers listen at this address.
- On a multiaccess network, a DR and BDR exist
and maintain adjacencies with all other OSPF routers on the network. If a DR
or BDR needs to send a link-state update, it will send it to all OSPF
routers at 224.0.0.5. However, the other routers on a multiaccess network
are adjacent only to the DR and the BDR and thus can send LSUs only to them.
For that reason, the DR and BDR have their own multicast address, 224.0.0.6.
Non-DR/BDR routers send their LSUs to 224.0.0.6, or "all DR/BDR
routers"

When the DR receives and acknowledges the LSU
destined for 224.0.0.6, it floods the LSU to all OSPF routers on the network at
224.0.0.5 .
Each router acknowledges receipt of the LSU with an LSAck.
If an OSPF router is connected to another
network, it floods the LSU to other networks by forwarding the LSU to the DR of
the multiaccess network, or to an adjacent router if in a point-to-point network
.
The DR, in turn, multicasts the LSU to the other OSPF routers in that network.
Upon receiving an LSU that includes new
information, an OSPF router updates its link-state database. It then runs the
SPF algorithm using the new information to recalculate the routing table. After
the SPF hold timer expires, the router switches over to the new routing table. 
If a route already exists in a Cisco router, the
old route is used while the SPF algorithm is calculating the new information. If
the SPF algorithm is calculating a new route, the router will not use that route
until after the SPF calculation is complete.
It is important to note that even if a change in
link state does not occur, OSPF routing information is periodically refreshed.
Each LSA entry has its own age timer. The default timer value is 30 minutes.
After an LSA entry ages out, the router that originated the entry sends an LSU
to the network to verify that the link is still active.
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