Planet XGS3-42000R User Manual page 287

The features of OSPF protocol include the following: OSPF supports networks of various scales, several
hundreds of layer3 switches can be supported in an OSPF network. Routing topology changes can be quickly
found and updating LSAs can be sent immediately, so that routes converge quickly. Link-state information is
used in shortest path algorithm for route calculation, eliminating loop route. OSPF divides the autonomous
system into areas, reducing database size, bandwidth occupation and calculation load. (According to the
position of layer3 switches in the autonomous system, they can be grouped as internal area switches, area
border switches, AS border switches and backbone switches). OSPF supports load balance and multiple
routes to the same destination of equal costs. OSPF supports 4 level routing mechanisms (process routing
according to the order of intra-area path, inter-area pat h, type 1 external path and type 2 external path). OSPF
supports IP subnet and redistribution of routes from the other routing protocols, and interface-based packet
verification. OSPF supports sending packets in multicast.
Each OSPF layer3 switch maintains a database describing the topology of the whole autonomous system.
Each layer3 switch gat hers the local status information, such as available int erface, reachable neighbors, and
sends link-state advertisement (sending out link-state information) to exchange link -state information with
other OSPF layer3 s witches to form a link-state database describing the whole autonomous system. Each
layer3 switch builds a short est path tree rooted by itself according to the link-state database, this tree provides
the routes to all nodes in an autonomous system. If two or more layer3 s witches exist (i.e. multi-access
network), "designat ed lay er3 s witch" 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.
OSPF protoc ol 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 t he network. OSPF uses four different kinds
of routes; they are int ra-area route, inter-area route, type 1 external route and type 2 external route, in the
order of highest priority to lowest. The route inside an area and bet ween areas describes the internal network
structure of an autonomous system, while 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 OSPF from the other interior routing protoc ols, the costs of those routes are comparable with
the costs of OSPF routes; the second type of exterior route corresponds to the information introduced by
OSPF from the other exterior routing prot ocols, but the costs of those routes are far greater than that of OSPF
routes, so OSPF route cost is ignored when calculating route costs.
OSPF areas are centered wit h 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 introduc ed 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, net work LSA, net work 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.
Rout er LSA is generated by each lay er3 s witch inside an OSPF area, and is sent to all the other neighboring
layer3 s witches in the same area; network LSA is generated by the designat ed layer3 s witch in the OSPF
area of multi-access network, and is sent to all other neighboring layer3 switches in t his 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
37-2