HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Routing Confi - Page 60

Area based OSPF network partition Network partition In a large OSPF routing domain, the LSDB becomes very huge and SPF computation consumes many storage and CPU resources. In addition, because topology changes can easily occur, OSPF packets generated for route information synchronization are enormous, occupying excessive bandwidth. To solve these problems, OSPF splits an AS into multiple areas, each of which is identified by an area ID. The boundaries between areas are routers rather than links. A network segment (or a link) can only reside in one area. An OSPF interface must be specified to belong to its attached area, as shown in Figure 13. Figure 13 Area based OSPF network partition After network partition, ABRs perform route summarization to reduce the number of LSAs advertised to other areas and minimize the effect of topology changes. Backbone area and virtual links Each AS has a backbone area that distributes routing information between none-backbone areas. Routing information between non-backbone areas must be forwarded by the backbone area. OSPF requires the following: • All non-backbone areas must maintain connectivity to the backbone area. • The backbone area itself must maintain connectivity. In practice, the requirements may not be satisfied due to lack of physical links. OSPF virtual links can solve this problem. A virtual link is established between two ABRs through a non-backbone area and is configured on both ABRs to take effect. The non-backbone area is called a transit area. In the following figure, Area 2 has no direct physical link to the backbone area 0. You can configure a virtual link between the two ABRs to connect Area 2 to the backbone area. 50