88
OSPF areas are centered with the Backbone area, identified as 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 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
flood 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 switches receives LSAs from other layer3
switches, and all LSAs are 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.