OSPF – Flooding

What is Flooding ?

To operate the OSPF protocol in a network, it is mandatory that each and every router in that network will have a clear-cut idea of the entire network topology. For that routers should send the update to each other. This process is called Flooding.

In short , Flooding is the process by which these changed or new LSAs are sent throughout the network, to
ensure that the database of every node is updated and remains identical to all other nodes’ databases.

Flooding makes use of the following two OSPF packet types:

  • Link State Update packets (type 4)
  • Link State Acknowledgment packets (type 5)

Each Link State Update and Acknowledgment packet may carry multiple LSAs.

On point-to-point networks, updates are sent to the multicast address AllSPFRouters ( On
point-to-multipoint and virtual link networks, updates are unicasted to the interface addresses of the
adjacent neighbors.
On broadcast networks, DRothers form adjacencies only with the DR and BDR. Therefore, updates are
sent to the address AllDRouters ( The DR in turn multicasts an Update packets containing the
LSA to all adjacent routers on the network using the address AllSPFRouters. All routers then flood the
LSA out all other interfaces . Although the BDR hears and records LSAs multicast from
DRothers, it will not reflood or acknowledge them unless the DR fails to do so. The same DR/BDR
functionality exists on NBMA networks, except that LSAs are unicast from DRothers to the DR and
BDR, and the DR unicasts a copy of the LSA to all adjacent neighbors.


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