User's Manual

39-2
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 OSPFv3 protocol include the following: OSPFv3 supports networks of various scales, several
hundreds of Layer3 switches can be supported in an OSPFv3 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. OSPFv3 divides the
autonomous system intro 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, edge switches and backbone switches). OSPFv3 supports load balance and
multiple routes to the same destination of equal costs. OSPFv3 supports 4 level routing mechanisms (process
routing according to the order of route inside an area, route between areas, type 1 external route and type 2
external route). OSPFv3 support IP subnet and redistribution of routes from the other routing protocols, and
interface-based packet verification. OSPFv3 supports sending packets in multicast.
Each OSPFV3 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 OSPFv3 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.
OSPFv3 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. OSPFv3 uses four different
kinds of routes: they are the route inside the area, route between areas, 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 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 OSPFv3 from the other interior routing protocols,
the costs of those routes are comparable with the costs of OSPFv3 routes; the second type of exterior route
corresponds to the information introduced by OSPFv3 from the other exterior routing protocols, but the costs
of those routes are far greater than that of OSPFv3 routes, so OSPFv3 route cost is ignored when calculating
route costs.
OSPFv3 areas are centered with the Backbone area, identified as the 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.