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OSPFv3 areas are centered with the Backbone area, identified as the Area 0, all the other
areas must be connected to Area 0 logically, and Area 0 must be continuous. For this reason, the
concept of virtual link is introduced to the backbone area, so that physically separated areas still
have logical connectivity to the backbone area. The configurations of all the layer3 switches in
the same area must be the same.
In one word, LSA can only be transferred between neighboring Layer3 switches, and
OSPFv3 protocol includes seven kinds of LSA: link LSA, internal-area prefix LSA, router LSA,
network LSA, inter-area prefix LSA, inter-area router LSA and autonomic system exterior LSA.
Router LSA is generated by each Layer 3 switch in an OSPF area, and is sent to all other
neighboring Layer 3 switch in this area; network LSA is generated by designated Layer 3 switch
in the OSPF area of multi-access network and is sent to all other neighboring layer3 switches in
this area.(To reduce data traffic among each Layer 3 switches in the multi-access network,
“designated layer3 switch” and “backup designated layer3 switch” should be selected in the
multi-access network, and the network link-state is broadcasted by designated Layer 3 switch);
the inter-area prefix LSA and inter-area router LSA are generated by OSPF area border Layer 3
switches and transferred among those switches. The autonomic system exterior LSA is
generated by autonomic system exterior border Layer 3 switches and transferred in the whole
autonomic system. Link LSA is generated by Layer 3 switch on the link and sent to other Layer 3
switches on the link. Internal-area prefix LSA is generated by designated layer3 switch of each
link in this area, and flooded to the whole area.
For autonomous system focused on exterior link-state announcement, OSPFv3 allow some
areas to be configured as STUB areas in order to reduce the size of topological database.
Router LSA, network LSA, inter-area prefix LSA, link LSA, internal-area prefix LSA are permitted
to advertise to STUB area. Default route must be used in STUB area, Layer 3 switches on the
area border of STUB area announces to default routes of STUB area by inter-area prefix LSA;
these default routes only flood in STUB area, not outside of STUB area. Each STUB area has a
corresponding default route, the route from STUB area to AS exterior destination depends only
on default route of this area.
The following simply outlines the route calculation process of OSPFv3 protocol:
a) Each OSPF-enabled layer3 switch maintains a database (LS database) describing the
link-state of the topology structure of the whole autonomous system. Each layer3 switch
generates a link-state advertisement according to its surrounding network topology structure
(router LSA), and sends the LSA to other layer3 switches through link-state update (LSU)
packets. Thus, each layer3 switches receives LSAs from other layer3 switches, and all LSAs
combined to the link-state database.
b) Since a LSA is the description of the network topology structure around a layer3 switch, the
LS database is the description of the network topology structure of the whole network. The
layer3 switches can easily create a weighted vector map according to the LS database.