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information to destination outside the autonomous system. The first type of exterior route
corresponds to the information introduced by OSPFv3 from the other interior routing
protocols, the costs of those routes are comparable with the costs of OSPFv3 routes; the
second type of exterior route corresponds to the information introduced by OSPFv3 from
the other exterior routing protocols, but the costs of those routes are far greater than that
of OSPFv3 routes, so OSPFv3 route cost is ignored when calculating route costs.
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:
1
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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