3com 5500G ユーザーズマニュアル

C

HAPTER

 15: R

OUTING

 C

ONFIGURATION

 G

UIDE

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.

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.

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.

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.

Figure 30   Network diagram for (Totally) stub area configuration

 

Run OSPF on the network devices. Configure a (totally) stub area to reduce the 
routing table size.

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 -int2 00
10.3.1.1 /24

Vlan -int200
10.3.1.2/24

Switch A

Vlan- int2 00
10 .2.1.1/24

Vlan- int3 00
10 .5.1.1/24

Switch D

Switch 5500

Release V03.02.04 

All versions