3com 5500-ei pwr Installation Instruction

 

4-15 

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 

the AS, you can configure these areas as stub areas.  

A stub area cannot redistribute any external route. For this reason the concept of NSSA is introduced. 

Type-7 LSAs can be advertised in an NSSA. Type-7 LSAs are generated by ASBRs in the NSSA, and 

will be transformed into Type-5 LSAs (AS-external LSAs) when reaching ABRs in the NSSA area, which 

will then be advertised to other areas. 

After area partition, the OSPF route updates between non-backbone areas are exchanged by way of 

the backbone area. Therefore, OSPF requires that all the non-backbone areas should keep connectivity 

with the backbone area and the backbone area must keep connectivity in itself. 

If the physical connectivity cannot be ensured due to various restrictions, you can configure OSPF 

virtual links to satisfy this requirement.  

Before configuring OSPF area attributes, perform the following tasks:  

z 

Configuring the network layer addresses of interfaces so that the adjacent nodes are reachable to 

each other at the network layer 

z 

Performing basic OSPF configuration 

Follow these steps to configure OSPF area attributes: 

Enter system view 

— 

Enter OSPF view 

ospf [ process-id [ router-id 
router-id ] ] 

— 

Enter OSPF area view 

area area-id

 

— 

Configure the current area to be 
a stub area 

stub [ no-summary ] 

Optional 

By default, no area is configured 
as a stub area. 

Configure the current area to be 
an NSSA area 

nssa 
[ default-route-advertise | 
no-import-route | 
no-summary | 
translate-always ] * 

Optional 

By default, no area is configured 
as an NSSA area. 

Configure the cost of the default 
route transmitted by OSPF to a 
stub or NSSA area 

default-cost cost 

Optional 

This can be configured on an ABR 
only. By default, the cost of the 
default route to a stub or NSSA 
area is 1.