Stub Area And Totally Stub Area; Not-So-Stubby Area; Ospf Authentication - Zte ZXR10 5900E Series User Manual

ZXR10 5900E Series User Manual (IPv4 Volume)
OSPF sends Link State Acknowledgements (LSAck) to make sure
that each LSA is received by neighbors. An LSAck contains the
head of the confirmed LSA, which is sufficient for identifying an
LSA uniquely. When a router sends an LSA to an interface, the
LSA is recorded in the resend queue of the interface. Router will
wait the preset time for the LSAck of the LSA. If it does not receive
the LSAck within the preset time, it will resend the LSA. A router
can send the original LSU by both unicast and multicast but can
resend the LSU only by unicast.

Stub Area and Totally Stub Area

A stub area is an area into which AS External LSAs are not flooded.
If type 5 LSAs are not known inside an area, type 4 LSAs are
unnecessary; these LSAs are blocked. ABRs at the edge of a stub
area will use Network Summary LSAs to advertise a single default
route into the area.
In a stub area, all routers must be configured as stub routers.
Hello Packet contains a "stub area" flag bit, which must be consis-
tent among neighbors.
ABR in a stub area can filter out type 5 LSAs to prevent them
from being advertised to the stub area. In addition, the ABR will
generate a type 3 LSA to advertise a default route to destination
addresses outside the AS.
ABR also filters out the Type 3 LSAs and advertises a default route
to destination addresses outside of the area, this area is called
totally stubby area.

Not-So-Stubby Area

Routers in a stub area do not allow type 5 LSAs so the ASBR is not
part of a stub area. To create a stub area with ASBR, routers in
this area receive from the ASBR the routes outside of the AS but
external routing information from other areas is blocked.
Not-so-stubby area (NSSA) allows external routes to be advertised
into the OSPF autonomous system while retaining the character-
istics of a sub area to the rest of the autonomous system. To
do this, the ASBR in an NSSA originates type 7 LSAs to advertise
the external destinations. These NSSA External LSAs are flooded
throughout the NSSA but are blocked at the ABR. On the other
hand, it converts type 7 LSAs into type 5 LSAs.

OSPF Authentication

Authentication applies to packet exchange between OSPF neigh-
bors. Neighbors must agree on the authentication type, which is
included in all packets.
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