H3C S7500 Series Operation Manual page 321

Operation Manual – Routing Protocol
H3C S7500 Series Ethernet Switches
space, complicate the running of SPF algorithm, and increase CPU load. Furthermore,
as a network grows larger, it is more potential to have changes in the network topology.
Hence, the network will often be in "turbulence", and a great number of OSPF packets
will be generated and transmitted in the network. This will lower the network
bandwidth utilization. In addition, each change will cause all the routers on the
network to recalculate routing information.
OSPF solves the above-mentioned problems by dividing an AS into multiple areas.
Areas group routers logically. A router on the border of an area belongs to more than
one area. A router connecting the backbone area to a non-backbone area is called an
area border router (ABR). An ABR can connect to the backbone area physically or
logically.
Area partition in OSPF reduces the number of LSAs in the network and enhances
OSPF scalability. To further reduce routing table size and the number of LSAs in some
non-backbone areas on the edge of an AS, you can configure these areas as stub
areas.
A stub area cannot import any external routes. For this reason the concept NSSA area
(not-so-stubby area) is introduced. In an NSSA area, type 7 LSAs are allowed to be
propagated. A type 7 LSA is generated by an ASBR (autonomous system boundary
router) in a NSSA area. A type 7 LSA reaching an ABR in the NSSA area is
transformed into an AS-external LSA, which is then advertised to other areas.
IV. Backbone area and virtual link
Backbone Area
In OSPF area partition, not all areas are parallel. One area, whose area ID is 0, is
different from all the other areas. This area is called the backbone area.
Virtual link
Since all areas must be connected to the backbone area, the concept virtual link is
introduced to maintain logical connectivity between the backbone area and any other
areas physically separated from the backbone area.
V. Route summary
After an AS is divided into different areas that are interconnected through OSPF
ABRs, the routing information between areas can be reduced through route summary.
This reduces the size of routing tables and improves the calculation speed of routers.
After an ABR in an area calculates the intra-area routes in the area, the ABR
aggregates multiple OSPF routes into one LSA (based on the summary configuration)
and sends the LSA outside the area.
For example, in
19.1.2.0/24, and 19.1.3.0/24. If route summary is configured, the three routes are
Figure 4-1, there are three intra-area routes in Area 19: 19.1.1.0/24,
4-3
Chapter 4 OSPF Configuration