User's Manual

38-3
In conclusion, LSA can only be transferred between neighboring Layer3 switches, OSPF protocol includes 5
types of LSA: router LSA, network LSA, network summary LSA to the other areas, ASBR summary LSA and
AS external LSA. They can also be called type1 LSA, type2 LSA, type3 LSA, type4 LSA, and type5 LSA.
Router LSA is generated by each Layer3 switch inside an OSPF area, and is sent to all the other neighboring
Layer3 switches in the same area; network LSA is generated by the designated Layer3 switch in the OSPF
area of multi-access network, and is sent to all other neighboring Layer3 switches in this area. (In order to
reduce traffic on Layer3 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 the designated Layer3 switch); network summary LSA is generated by border switches in an
OSPF area , and is transferred among area border Layer3 switches; AS external LSA is generated by Layer3
switches on external border of AS, and is transferred throughout the AS.
As to autonomous systems mainly advertises exterior link-state, OSPF allow some areas to be configured as
STUB areas to reduce the size of the topology database. Type4 LSA (ASBR summary LSA) and type5 LSA
(AS external LSA) are not allowed to flood into/through STUB areas. STUB areas must use the default routes,
the Layer3 switches on STUB area edge advertise the default routes to STUB areas by type 3 summary LSA,
those default routes only floods inside STUB area and will not get out of STUB area. Each STUB area has a
corresponding default route, the route from a STUB area to AS exterior destination must rely on the default
route of that area.
The following simply outlines the route calculation process of OSPF 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 switch
receives LSAs from other Layer3 switches, and all LSAs are combined to the link-state database.
2） Since an 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 that
broadcasts it, so that additional information about the autonomous system can be recorded. As a
result, the route table of each Layer3 switch is different.
OSPF protocol is developed by the IETF. The OSPF v2 widely used now is fulfilled according to the content
described in RFC2328.