106 CHAPTER 15: ROUTING CONFIGURATION GUIDE
Configuring a (Totally) Stub Area
When a large number of OSPF routers are present on a network, the LSDB of
routers may become so large that a great amount of storage space is occupied
and CPU resources are exhausted when performing the SPF computation.
In addition, as the topology of a large network is prone to changes, enormous
OSPF packets may be created, reducing bandwidth utilization. Each topology
change makes all the routers perform a route recalculation.
To address this issue, OSPF divides an AS into multiple areas.
Backbone area
The area ID of the backbone area is 0. The backbone area is responsible for
distributing routing information between none-backbone areas. Routing
information of non-backbone areas must be forwarded by the backbone area.
(Totally) Stub area
The ABR in a stub area does not distribute Type-5 LSAs into the area, so the
routing table size in this area is reduced significantly.
To further reduce the routing table size in a stub area, you can configure the stub
area as a totally stub area, where the ABR advertises neither the addresses of other
areas nor the external routes.
NSSA area
Similar to a stub area, a Not So Stubby Area (NSSA) area imports no Type-5 LSAs
but can import Type-7 LSAs that are generated by the ASBR and distributed
throughout the NSSA area. After reaching the NSSA ABR, Type-7 LSAs are
translated into Type-5 LSAs by the ABR for advertisement to other areas.
Network Diagram Figure30 Network diagram for (Totally) stub area configuration
Networking and
Configuration
Requirements
Run OSPF on the network devices. Configure a (totally) stub area to reduce the
routing table size.
Applicable Products
Area 0
Area 1
Stub
Area 2
Switch C
Vlan-int100
10.1.1.2/24
Vlan-int100
10.1.1.1/24
Vlan-int300
10.4.1.1/24
Vlan-int200
10.2.1.2/24
Switch B
Vlan-int200
10.3.1.1/24
Vlan-int200
10.3.1.2/24
Switch A
Vlan-int200
10.2.1.1/24
Vlan-int300
10.5.1.1/24 Switch D
ASBR
Product series Software version Hardware version
Switch 5500 Release V03.02.04 All versions