498
external route and second category 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, first category external route and second
category 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 external 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 external route corresponds to the information introduced by OSPFv3 from
the other external 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.
In one word, LSA can only be transferred between neighboring Layer3 switches,
and OSPFv3 protocol includes seven kinds of LSA: link LSA,intra-area prefix LSA, router
LSA, network LSA, inter-area prefix LSA, inter-area router LSA and Autonomous system
external LSA. Router LSA is generated by each Layer 3 switch in an OSPF area, and is
sent to all other neighboring Layer 3 switch in this area; network LSA is generated by
