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Chapter 4
OSPF Areas
You should remember that the previous process occurs when multiple
routers become active at the same time on a segment. If a DR and BDR
already exist on the segment, any new interfaces accept the DR and BDR
regardless of their own Router ID or Router Priority.
To further the example, if initially there is only one OSPF router interface
active on the segment, it becomes the DR. The next router would become the
BDR. Subsequent routers would all accept the existing DR and BDR and
form adjacencies with them.
LSA Flooding
LSA flooding is the method by which OSPF shares routing information. Via
LSU packets, LSA information containing link-state data is shared with all
OSPF routers. The network topology is created from the LSA updates.
Flooding is used so that all OSPF routers have the topology map from which
SPF calculations may be made.
Efficient flooding is achieved through the use of a reserved multicast
address, (AllSPFRouters). LSA updates (indicating that something
in the topology changed) are handled somewhat differently. The network
type determines the multicast address used for sending updates. Table 4.2
contains the multicast address associated with LSA flooding. Point-to-
multipoint networks use the adjacent router's unicast IP address. Figure 4.3
depicts a simple update and flood scenario on a broadcast multi-access network.
T A B L E 4 . 2
LSA Update Multicast Addresses
Network Type
Multicast Address
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