PLANET XGS3-24042 - Manual (Page 306)

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One major advantage of link-state routing protocols is the fact that infinite counting is 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
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 describes 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.
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.