Dell N2000 Series, Configuration Manual

Page: 1118 / 1460

1118
Configuring OSPF and OSPFv3
Flood Blocking
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.