LSA Flooding

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, 224.0.0.5 (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 5.2
contains the multicast address associated with LSA flooding. Networks that do not natively support
broadcasts, such as point-to-multipoint non-broadcast networks, use the adjacent router’s unicast IP
address. Figure 5.3 depicts a simple update and flood scenario on a broadcast multi-access network.