H3C SecPath F1800-A Operation Manual page 265

Operation Manual - Network and Routing Protocol
H3C SecPath F1800-A Firewall
also exchanged between them. Once a DR becomes invalid, the BDR will turn into a
DR instantly.
III. Area
As the network keeps extending in scale, if more and more routers in a network run
OSPF, LSDB will become very huge. As a result, a great amount of memory is
occupied and much CPU is consumed to complete SPF algorithm. In addition,
network expansion makes it more possible to change topology. As a result, many
OSPF packets are forwarded in the network, and bandwidth utility of the network is
reduced.
To solve this problem, OSPF divides AS into several areas. Areas divides a router into
groups logically. Each area is marked by area ID. One of the most important areas is
area 0, which is also named backbone area.
The router that connects backbone area and non-backbone area is named Area
Border Router (ABR).
IV. Route Aggregation
AS is divided into different areas, each of which is interconnected through OSPF ABR.
The routing information between areas can be reduced through route aggregation.
Thus, the size of routing table can be reduced and the calculation speed of the router
can be improved.
After calculating an intra-area route in an area, the ABR will look up the routing table
and encapsulate each OSPF route into an LSA and send it outside the area.
For example, in
19.1.1.0/24, 19.1.2.0/24 and 19.1.3.0/24. If route aggregation is configured and the
three routes are aggregated into one route 19.1.0.0/16, only one LSA, which
describes the route after aggregation, is generated on RTA.
Figure 5-1, there are three intra-area routes in area 19, which are
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Chapter 5 OSPF Configuration