Huawei Quidway NetEngine80E Configuration Manual page 286

HUAWEI NetEngine80E/40E Router
Configuration Guide - IP Routing
Process of OSPF Route Calculation
The process of calculating OSPF routes is as follows:
1.
2.
3.
4.
Area Division
The number of routers increases with the unceasing expansion of the network scale. This leads
to a large LSDB on each router. As a result, the load of each router is very heavy. OSPF solves
this problem by dividing an AS into different areas. An area is regarded as a device group
logically. Each group is identified by an area ID. On the border of an area resides a router rather
than a link. A network segment (or a link) belongs to only one area. That is, the area to which
each OSPF interface belongs must be specified, as shown in
Figure 5-1 OSPF area division
After area division, route aggregation can be performed on border routers to reduce the number
of LSAs advertised to other areas. Route aggregation also minimizes the influence caused by
changes in the topology.
Issue 02 (2014-09-30)
Based on the surrounding network topology, each OSPF device originates a Link State
Advertisement (LSA). The router then transmits Update packets containing the LSAs to
other OSPF devices.
Each OSPF device collects the LSAs from other devices, and all these LSAs compose the
LSDB. An LSA describes the network topology around a router, whereas an LSDB
describes the network topology of the whole AS.
OSPF devices transform the LSDB into a weighted directed map. The weighted directed
map reflects the topology of the entire network. All routers in the same area have the same
map.
According to the directed map, each router uses the Shortest Path First (SPF) algorithm to
calculate the shortest path tree, regarding itself as the root. The tree displays the routes to
each node in the AS.
Area1
Area2
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Figure
Area4
Area0
Area3
5 OSPF Configuration
5-1.
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