Chapter 14 OSPF Configuration
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accurate and complete network diagram and generate routes
used in its routing table. ZTE ZXR10 GER supports following six
types of LSAs:
Type 1: Router LSA
Type 2: Network LSA
Type 3: Network summary LSA
Type 4: ASBR summary LSA
Type 5: External LSA of AS
Type 7: External LSA of NSSA A
The operation of OSPF depends upon all routers sharing same
common link state database in one area. Therefore, all LSAs are
flooded via this area and processing must be reliable. Each
router receiving LSAs in a special area will flood LSAs to other
interfaces in area. LSAs do not have their own packets, and they
are included in Link state Update (LSU) packets.
Several LSAs can be included in same LSU. When a router
receives an LSU, this does not send out the packets simply, but
separates the packets from LSA and inputs them to its database.
In addition, the router will construct its own LSU and send the
updated LSU to the neighbor router(s).
OSPF uses Link State Acknowledgement (LSAck) to confirm
whether each LSA is successfully received by its neighbor. An
LSAck has header of an acknowledged LSA which provides
sufficient information for uniquely identifying an LSA. When a
router sends an LSA to an interface, the LSA will be recorded in
the retransmission queue of interface.
The router will wait for maximum time interval to receive the
LSAck of LSA. If the router does not receive the LSAck in
specified time, the router will retransmit the LSA. The router can
send the original LSU in unicast or multicast mode, but the
retransmitted LSU is in unicast mode.
If a non-backbone area does not have an ASBR, a router only
has one path to an AS external network, that is, through an ABR.
Thus, routers in these areas send LSAs sent to an unknown host
outside the AS to ABR. Therefore, LSAs of type 5 do not need to
be flooded to area and also the area does not have LSAs of type
4. Such an area type is called a stub area.
In a stub area, all routers must be configured as stub routers. A
Hello packet contains a "stub area" flag bit. The flag bit must be
consistent among neighbors.
An ABR in a stub area can filter LSAs of type 5 to prevent them
from being advertised to stub area. In meantime, the ABR will
generate an LSA of type 3 to advertise a default route to a
destination address external to the AS.
If the ABR also filters the LSA of type 3 and also advertises a
default route to the destination address external to an area,
such an area is called a totally stubby area.
OSPF
Operation
LSU
Link State
Acknowledgem
ent
Stub Area
Totally Stub
Area
