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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 external LSA is generated by autonomic system
external 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 external 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 external destination depends only on default route of this area.
The following simply outlines the route calculation process of OSPFv3 protocol:
1
)
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) packages. Thus, each layer3 switches receives LSAs from other layer3
switches, and all LSAs combined to the link-state database.
2
)
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. Obviously, all layer3 switches in the same autonomous system will have the
same network topology map.
3
)
Each layer3 switch uses the shortest path finding (SPF) algorithm to calculate a tree of
shortest path rooted by itself. The tree provides the route to all the nodes in the
autonomous system, leaf nodes consist of the external route information. The external
route can be marked by the layer3 switch broadcast it, so that additional information
about the autonomous system can be recorded. As a result, the route table of each
layer3 switch is different.
OSPFv3 protocol is developed by the IETF, the OSPF v3 used now is fulfilled