3Com 4210, 5500G, 4200G manual Backbone area, Totally Stub area, Nssa area

106CHAPTER 15: ROUTING CONFIGURATION GUIDE

Configuring a (Totally) When a large number of OSPF routers are present on a network, the LSDB of

Stub Arearouters 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 Figure 30	Network diagram for (Totally) stub area configuration
Switch A				Area 0	Switch B
			Vlan- int100
		10.1.1.1/24
					Vlan-int100
Vlan-int200					10.1 .1.2/24	Vlan-int200
10.2.1.1/24						10.3.1.1/24
Area 1	Vlan-int200				Area 2	Vlan-int200
						10.3.1.2/24
Stub	10	.2.1.2/24
						ASBR
		Vlan-int300			Vlan- int300
Switch C		10.4.1.1/24			10.5.1.1/24	Switch D

Networking and	Run OSPF on the network devices. Configure a (totally) stub area to reduce the
Configuration	routing table size.
Requirements
Applicable Products
	Product series	Software version	Hardware version
	Switch 5500	Release V03.02.04	All versions