Extreme Networks Summit1 - Manual (Page 155)

S UMMIT S WITCH I NSTALLATION AND U SER G UIDE 10-7
O VERVIEW OF OSPF
V IRTUAL L INKS
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.
Figure 10-2: Virtual link for stub area
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.
Figure 10-3: Virtual link providing redundancy
Area 0
ABR
ABR
Area 2
Area 1
Virtual link
Area 0
Area 2 Area 1 Area 3
ABR 2 ABR 1
Virtual link
Summit.bk : 10PROT.FM Page 7 Thursday, June 18, 1998 9:27 AM