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advantages release some layer3 switch resources, as the process ability and bandwidth used by
bad route information are minor.
The features of OSPF protocol include the following: OSPF supports networks of various
scales, several hundreds of layer3 switches can be supported in an OSPF network. Routing
topology changes can be quickly found and updating LSAs can be sent immediately, so that
routes converge quickly. Link-state information is used in shortest path algorithm for route
calculation, eliminating loop route. OSPF divides the autonomous system into areas, reducing
database size, bandwidth occupation and calculation load. (According to the position of layer3
switches in the autonomous system, they can be grouped as internal area switches, area border
switches, AS border 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 intra-area path, inter-area path, type 1
external path and type 2 external path). OSPF supports 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 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 provides 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 intra-area route, inter-area route,
type 1 external route and type 2 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 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.