Extreme Networks Summit1 manual 2Virtual link for stub area, 3Virtual link providing redundancy

OVERVIEW OF OSPF

VIRTUAL LINKS

In the situation when a new area is introduced that does have a direct physical attachment to the backbone, a virtual link is used. A virtual link provides a logical path between the ABR of the disconnected area and the ABR of the backbone. A virtual link must be established between two ABRs that have a common area, with one ABR connected to the backbone. Figure 10-2 illustrates a virtual link.

Area 2

ABR

ABR

Area 1

Virtual link

Figure 10-2:Virtual link for stub area

Area 0

Virtual links are also used to repair a discontiguous backbone area. For example, in Figure 10-3, if the connection between ABR1 and the backbone fails, the connection using ABR2 provides redundancy so that the discontiguous area can continue to communicate with the backbone using the virtual link.

Area 0

ABR 1ABR 2

Area 1 Area 2Area 3

Virtual link

Figure 10-3:Virtual link providing redundancy

SUMMIT SWITCH INSTALLATION AND USER GUIDE 10-7