Flooding Process Protocol
Flooding in OSPF is responsible for validating and distributing link-state updates to the link-state database whenever a change or update occurs to a link. Changes or updates are key concepts regarding when flooding occurs. Flooding is part of the LSDB synchronization mechanism within OSPF. The goal of this mechanism is to keep the LSDBs of the routers in an OSPF domain synchronized within time in the presence of topological changes.
In the event of a link-state change (for example, from up to down), the router that experienced the change transmits a flooding packet (link-state update), which contains the state change. This update is flooded out all the routers' interfaces. The primary goal of flooding is to ensure that every router receives the changed or updated LSA within the flooding scope (area or domain).
Flooding occurs differently between neighbors in OSPF depending on a the following factors:
• Type 1 through 4 and 7 LSAs are flooded within an area; each LSA has a different flooding scope, as previously discussed.
• Type 5 LSAs are flooded throughout the OSPF domain, except for stub and NSSAs.
• When a DR is present, only non-DRs flood to the DR. The DR then floods to everyone as required.
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• When two OSPF routers have not yet established an adjacency, they do not flood each other; that is, they are in the middle of LSDB synchronization.
OSPF expects an acknowledgment from each link-state update. To ensure that the flooded packet is received by all of its neighbors, OSPF continues to retransmit the link-state update packet until it receives an acknowledgment from each of its neighbors. OSPF can acknowledge an update in two ways:
• When the destination router sends an acknowledgment directly to the source router—In this case, there is no DR in use by OSPF if this is occurring.
• When a DR is in use and it receives the update—When this occurs, the DR immediately retransmits this update to all other routers. Therefore, when the sending router hears this retransmission, it is considered an acknowledgment, and no further action is taken.
Figure 3-25 shows the field names and packet layout for the flooding subprotocol.
Figure 3-25 Flooding Protocol Packet Layout
0 12 3
0 1234567890 1234567890 1234567890 1
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Continue reading here: Understanding LSA Group Pacing
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