PLANET XGS3-24040 - Manual (Page 298)

Chapter 38 OSPFv3
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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
exterior border Layer 3 switches and transferred in the whole autonomic system. Link LSA is generated by
Layer 3 switch on the link and sent to other Layer 3 switches on the link. Internal-area prefix LSA is generated
by designated layer3 switch of each link in this area, and flooded to the whole area.
For autonomous system focused on exterior link-state announcement, OSPFv3 allow some areas to be
configured as STUB areas in order to reduce the size of topological database. Router LSA, network LSA,
inter-area prefix LSA, link LSA, internal-area prefix LSA are permitted to advertise to STUB area. Default route
must be used in STUB area, Layer 3 switches on the area border of STUB area announces to default routes
of STUB area by inter-area prefix LSA; these default routes only flood in STUB area, not outside of STUB
area. Each STUB area has a corresponding default route, the route from STUB area to AS exterior
destination depends only on default route of this area.
The following simply outlines the route calculation process of OSPFv3 protocol:
1. Each OSPF-enabled layer3 switch maintains a database (LS database) describing the link-state of the
topology structure of the whole autonomous system. Each layer3 switch generates a link-state
advertisement according to its surrounding network topology structure (router LSA), and sends the LSA
to other layer3 switches through link-state update (LSU) packets. Thus, each layer3 switches receives
LSAs from other layer3 switches, and all LSAs combined to the link-state database.
2. Since a LSA is the description of the network topology structure around a layer3 switch, the LS database
is the description of the network topology structure of the whole network. The layer3 switches can easily
create a weighted vector map according to the LS database. Obviously, all layer3 switches in the same
autonomous system will have the same network topology map.
3. Each layer3 switch uses the shortest path first (SPF) algorithm to calculate a tree of shortest path rooted
by itself. The tree provides the route to all the nodes in the autonomous system, leaf nodes consist of the
exterior route information. The exterior route can be marked by the layer3 switch broadcast it, so that
additional information about the autonomous system can be recorded. As a result, the route table of
each layer3 switch is different.
OSPFv3 protocol is developed by the IETF, the OSPF v3 used now is fulfilled according to the content
described in RFC2328 and RFC2740.