As shown above, a large-scale network is divided into three areas: Area 0, Area 1 and Area 2.
Area 1 and Area 2 exchange the routing information via Backbone Area, which has to maintain
its network connectivity at all time. The non-backbone Area 1 and Area 2 cannot communicate
directly with each other, but they can exchange routing information through the backbone Area
0. On large-scale networks, an appropriate area partition can help greatly to save network
resources and enhance the speed of the routing.
After the area partition in the network, routers of different type need to accomplish different
tasks. Different areas need to transmit the routing information to the backbone area in different
ways, due to their different locations relative to the backbone area. In the following, we will
introduce the details involved after the area partition.
1.
Router Type
As Figure 10-35 shows, after the area partition of the network, the routers need to accomplish
different tasks due to their locations in different areas, according to which the routers can be
classified into 4 types: Internal Router (IR), Backbone Router (BR), Area Boundary Router (ABR)
and Autonomous System Boundary Router (ASBR).
Figure 10-37 Classification of Routers
Responsibilities of different routers divide as Table 10-2.
Router
Name
Features
Responsibility
IR
All the routing
interfaces belong to
the same area
Flood and exchange its all link and interface information
with the adjacent routers in the same area, thus to
synchronize the link state database with the intra-area
routers.
BR
At least one routing
interface belongs to
the backbone area
Summarize the routing topology information from all areas
in AS via ABR and forward the communication data for all
areas.
ABR
Connect one or more
areas to the backbone
area
Maintain independent link state databases for different
areas, and deliver the topology information of each area to
the other areas via the backbone area.
209