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by the designated layer3 switch); summary LSA is generated by edge switches in an
OSPF area , and is transferred among area edge layer3 switches; AS external LSA is
generated by layer3 switches on external edge of AS, and is transferred throughout the
AS.
As to autonomous systems mainly advertises external link-state, OSPF allow some areas
to be configured as STUB areas to reduce the size of the topology database. Type4 LSA
(ASBR summary LSA) and type5 LSA (AS external LSA) are not allowed to flood
into/through STUB areas. STUB areas must use the default routes, the layer3 switches
on STUB area edge advertise the default routes to STUB areas by type 3 summary LSA,
those default routes only flood inside STUB area and will not get out of STUB area. Each
STUB area has a corresponding default route, the route from a STUB area to AS external
destination must rely on the default route of that area.
The following simply outlines the route calculation process of OSPF 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.
OSPF protocol is developed by the IETF, the OSPF v2 widely used now is fulfilled
according to the content described in RFC2328.
14.6.2 OSPF Configuration Task List
The OSPF configuration for
SMC Networks
series switches may be different from the
