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the autonomous system. The first type of exterior route corresponds to the information
introduced by OSPF from the other interior routing protocols, the costs of those routes
are comparable with the costs of OSPF routes; the second type of exterior route
corresponds to the information introduced by OSPF from the other exterior routing
protocols, but the costs of those routes are far greater than that of OSPF routes, so
OSPF route cost is ignored when calculating route costs.
OSPF areas are centered with the Backbone area, identified as 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 conclusion, LSA can only be transferred between neighboring Layer3 switches, OSPF
protocol includes 5 types of LSA: router LSA, network LSA, summary LSA to the other
areas, general LSA to AS edge switches and exterior AS LSA. They can also be called
type1 LSA, type2 LSA, type3 LSA, type4 LSA, and type5 LSA. Router LSA is generated
by each layer3 switch inside an OSPF area, and is sent to all the other neighboring
layer3 switches in the same area; network LSA is generated by the designated layer3
switch in the OSPF area of multi-access network, and is sent to all other neighboring
layer3 switches in this area. (In order to reduce traffic on layer3 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 the designated layer3 switch); summary LSA is generated by edge switches in an
OSPF area , and is transferred among area edge layer3 switches; AS exterior LSA is
generated by layer3 switches on exterior edge of AS, and is transferred throughout the
AS.
As to autonomous systems mainly advertises exterior 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 exterior 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 exterior
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
