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available interface, reachable neighbors, and sends link-state advertisement (sending out
link-state information) to exchange link-state information with other OSPF layer3 switches
to form a link-state database describing the whole autonomous system. Each layer3
switch builds a shortest path tree rooted by itself according to the link-state database, this
tree provide the routes to all nodes in an autonomous system. If two or more layer3
switches exist (i.e. multi-access network), "designated layer3 switch” and “backup
designated layer3 switch” will be selected. Designated layer3 switch is responsible for
spreading link-state of the network. This concept helps reducing the traffic among the
Layer3 switches in multi-access network.
OSPF protocol requires the autonomous system to be divided into areas. That is to
divide the autonomous system into 0 area (backbone area) and non-0 areas. Routing
information between areas are further abstracted and summarized to reduce the
bandwidth required in the network. OSPF uses four different kinds of routes; they are the
route inside the area, route between areas, first category external route and second
category external route, in the order of highest priority to lowest. The route inside an area
and between areas describe the internal network structure of an autonomous system,
while external routes describe how to select the routing information to destination outside
the autonomous system. The first type of external 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 external route
corresponds to the information introduced by OSPF from the other external 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 external 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
