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DES-7200 Configuration Guide Chapter 5 OSPF
Configuration
5-8
mutual independence way. These parts are called Areas. The backbone part is
called Backbone Area and always indicated by the numerical value 0 (or 0.0.0.0).
By using this hierarchical structure, each device is allowed to keep the link state
database in the area where it resides and the topology inside the area is invisible
to outside. In this way, the link state database of each device can be always in a
reasonable size, route calculation time is not too much and the number of
messages is not too big.
In the OSPF, the following types of special areas have been defined to meet
actual needs:
stub Area.
If an area is at the end part of the whole network, then we can design the area as
a stub area.
A stub area cannot learn the external route information of an AS (type 5 LSAs). In
practical application, external route information is very important in the linkstate
database. Therefore, the devices inside a stub area will learn little route
information, reducing the system resources for running the OSPF protocol.
When a device inside a stub area wants to access outside of an AS, use the
default route entrie (type3 LSA) generated from the default route information
published by Area Border Routers in the stub area.
NSSA area (Not-So-Stubby Area)
NSSA extends the stub area. By preventing from flooding type 5 LSAs to the
devices in the NSSA, it reduce the consumption of device resources. However,
unlike a stub area, it allows a certain amount of external route information of the
AS to enter an NSSA in other ways, namely, inject into the NSSA in the form of
type 7 LSAs.
To configure OSPFv3 area parameters, execute the following command in the
OSPFv3 configuration mode:
Command Function
area area-id stub
[no-summary]
Configure a stub area.
no-summary: configure the area to a totally stub
area, preventing the area border router in the stub
area from sending type3 and type4 LSAs to the
stub area.
area area-id default-cost cost
Configure the cost of the default route sent to a stub
area or NSSA.
To remove the configuration, use the no form of the above commands.