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impossible, this is because of the way link-state routing protocols build up their routing
table. The second advantage is that converging in a link-state interconnected network is
very fast, once the routing topology changes, updates will be flooded throughout the
network very soon. Those 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 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 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 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 how to select the routing information to destination outside
