DELL N3000 User Manual

Configuring OSPF and OSPFv3

OSPF is a link state routing protocol. Routers describe their local 

environment in Link State Advertisements (LSAs), which are distributed 

throughout an area or OSPF domain. Through this process, each router learns 

enough information to compute a set of routes consistent with the routes 

computed by all other routers. 
Normally, OSPF floods an LSA on all interfaces within the LSA's flooding 

scope. Flooding ensures that all routers receive all LSAs. A router normally 

receives a duplicate copy of each LSA once on each interface in the LSA's 

flooding scope. The duplicate deliveries make OSPF LSA distribution robust, 

but in highly interconnected networks, can cause a lot of buffer and CPU 

usage. Buffer and CPU use can be reduced by selectively blocking LSA 

flooding on some interfaces, while ensuring that LSAs are flooded on enough 

interfaces to guarantee delivery of all LSAs to all routers. When enabling 

flood blocking, the network administrator must ensure there is sufficient LSA 

flooding even when there are router and link failures. 
This feature enables a network administrator to disable LSA flooding on an 

interface. 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 on 

other, unblocked interfaces, and eventually reach neighbors on blocked 

interfaces. An LSA with interface flooding scope cannot be blocked; there is 

no other way for interface-scope LSAs to reach neighbors on the blocked 

interface. Allowing interface-scope LSAs on blocked interfaces allows graceful 

restart to work, even if the restarting router has neighbors on flood blocked 

interfaces.
When an interface is blocked, LSAs with area or AS scope are not sent to any 

neighbor on that interface. When flood blocking is enabled, OSPF does not 

advertise any LSAs with area or AS scope in its database description packets 

sent to neighbors on a blocked interface. When OSPF receives an LSA from a 

neighbor and the local database copy is newer than the received LSA, OSPF 

normally sends the newer LSA directly to the neighbor. If the neighbor is on a 

blocked interface, OSPF neither acknowledges the LSA nor sends the newer 

LSA. Instead, OSPF expects that the neighbor will receive the newer LSA 

indirectly.
Flooding is enabled by default.