938 Configuring OSPF and OSPFv3Flood BlockingOSPF is a link state routing protocol. Routers describe their localenvironment in Link State Advertisements (LSAs), which are distributedthroughout an area or OSPF domain. Through this process, each router learnsenough information to compute a set of routes consistent with the routescomputed by all other routers.Normally, OSPF floods an LSA on all interfaces within the LSA's floodingscope. Flooding ensures that all routers receive all LSAs. A router normallyreceives a duplicate copy of each LSA once on each interface in the LSA'sflooding scope. The duplicate deliveries make OSPF LSA distribution robust,but in highly interconnected networks, can cause a lot of buffer and CPUusage. Buffer and CPU use can be reduced by selectively blocking LSAflooding on some interfaces, while ensuring that LSAs are flooded on enoughinterfaces to guarantee delivery of all LSAs to all routers. When enablingflood blocking, the network administrator must ensure there is sufficient LSAflooding even when there are router and link failures.This feature enables a network administrator to disable LSA flooding on aninterface. Flood blocking only affects flooding of LSAs with area or AS (i.e.,domain-wide) scope. Such LSAs are expected to be flooded to neighbors onother, unblocked interfaces, and eventually reach neighbors on blockedinterfaces. An LSA with interface flooding scope cannot be blocked; there isno other way for interface-scope LSAs to reach neighbors on the blockedinterface. Allowing interface-scope LSAs on blocked interfaces allows gracefulrestart to work, even if the restarting router has neighbors on flood blockedinterfaces.When an interface is blocked, LSAs with area or AS scope are not sent to anyneighbor on that interface. When flood blocking is enabled, OSPF does notadvertise any LSAs with area or AS scope in its database description packetssent to neighbors on a blocked interface. When OSPF receives an LSA from aneighbor and the local database copy is newer than the received LSA, OSPFnormally sends the newer LSA directly to the neighbor. If the neighbor is on ablocked interface, OSPF neither acknowledges the LSA nor sends the newerLSA. Instead, OSPF expects that the neighbor will receive the newer LSAindirectly.Flooding is enabled by default.
