Accton Technology ES4626 - Manual (Page 485)

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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 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 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.
In one word, LSA can only be transferred between neighboring Layer3 switches,
and OSPFv3 protocol includes seven kinds of LSA: link LSA, internal-area prefix LSA,
router LSA, network LSA, inter-area prefix LSA, inter-area router LSA and autonomic
system exterior 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 designated Layer 3 switch in the OSPF area of multi-access network and is sent to all
other neighboring layer3 switches in this area.(To reduce data traffic among each Layer 3
switches in the multi-access network, “designated layer3 switch” and “backup designated
layer3 switch” should be selected in the multi-access network, and the network link-state
is broadcasted by designated Layer 3 switch); the inter-area prefix LSA and inter-area
router LSA are generated by OSPF area border Layer 3 switches and transferred among
those switches. The autonomic system exterior LSA is generated by autonomic system