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autonomous system, they can be grouped as internal switches, edge switches, AS edge
switches and backbone switches). OSPF supports load balance and multiple routes to
the same destination of equal costs. OSPF supports 4 level routing mechanisms
(process routing according to the order of route inside an area, route between areas, first
category exterior route and second category exterior route). OSPF support IP subnet and
redistribution of routes from the other routing protocols, and interface-based packet
verification. OSPF supports sending packets in multicast.
Each OSPF layer3 switch maintains a database describing the topology of the whole
autonomous system. Each layer3 switch gathers the local status information, such as
available interface, reachable neighbors, and sends link-state advertisement (sending
out link-state information) to exchange link-state information with the other OSPF layer3
switches to formal a link-state database describing the whole autonomous system. Each
layer3 switch builds a shortest path tree rooted in itself according to the link-state
database, this tree provide the route to all nodes in an autonomous system. If 2 or more
layer3 switches exist (multi-access to the network), "designated layer3 switch” and
“backup designated layer3 switch” will be selected. Designated layer3 switch is
responsible for broadcasting link-state of the network. This concept helps the traffic
among the switches.
OSPF protocol requires the autonomous system to be divided into areas. That is to
divide the
autonomous system into 0 field (back field) and non-0 filed. 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 exterior route and second category exterior 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 the routing information to destination outside 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 fair to the costs of OSPF routes;
the second type of exterior route corresponds to the information introduced by OSPF
from the other interior routing protocols, but the costs of those routes are far greater than
that of OSPF routes, and OSPF route cost is ignored when calculating route costs.
OSPF areas are centered with the Backbone area, identified as the 0 area, all the other
areas must be connected to the 0 area logically, and the 0 area must be online. For this
reason, the concept of virtual connection is introduced to the backbone area, so that
physically separated areas still have logical connectivity to this 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; network LSA is generated by the specified layer3 switch in the OSPF area of
multi-access network, and is sent to all the other neighboring layer3 switches. (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 switches in OSPF area edge, 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