HP 6125XLG R2306-HP 6125XLG Blade Switch Layer 3 - IP Routing Configuration Gu - Page 61

Figure 13 Area-based OSPF network partition Area 1 Area 4 Area 0 Area 2 Area 3 Backbone area and virtual links Each AS has a backbone area that distributes routing information between non-backbone areas. Routing information between non-backbone areas must be forwarded by the backbone area. OSPF includes the following requirements: • All non-backbone areas must maintain connectivity to the backbone area. • The backbone area must maintain connectivity within itself. In practice, these requirements might not be satisfied due to lack of physical links. OSPF virtual links can resolve this issue. A virtual link is established between two ABRs through a non-backbone area. It must be configured on both ABRs to take effect. The non-backbone area is called a transit area. In Figure 14, 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. Figure 14 Virtual link application 1 Virtual links can also be used to provide redundant links. If the backbone area cannot maintain internal connectivity due to the failure of a physical link, you can configure a virtual link to replace the failed physical link, as shown in Figure 15. 50