WB1 6.16 OSPF Flooding Reduction

Configure R5, SW2, and SW4 so that links in OSPF area 3 do not participate in periodic paranoid update LSA flooding.

在R5, SW2, and SW4每一個介面上，下指令：

範例：
interface FastEthernet0/0
  ip ospf flood-reduction

Per RFC 2328, OSPF Version 2, “an LSA's LS age is never incremented past the value MaxAge." When the Link State Age reaches MaxAge "the router must attempt to flush the LSA... by reflooding the MaxAge LSA just as if it was a newly originated LSA".

In Cisco’s IOS implementation of OSPF the MaxAge value is 3600 seconds, or 30 minutes. This means that each LSA is reflooded after 30 minutes, regardless if the topology is stable. This periodic flooding behavior is commonly referred to as the “paranoid update”. The ip ospf flood-reduction feature stops unnecessary LSA flooding by setting the DoNotAge (DNA) bit in the LSA, removing the requirement for the periodic refresh.

In the below output SW4 has the (DNA) field next to all LSAs learned from area 3, which indicates that they do not need to be periodically flooded.