Virtual Links Example 1 - D-Link NetDefend DFL-210 User Manual

4.4.2. OSPF
Virtual links are used for:
• Linking an area that does not have a direct connection to the backbone.
• Linking the backbone in case of a partitioned backbone.
Area without direct connection to the backbone
The backbone always need to be the center of all other areas. In some rare case where it is im-
possible to have an area physically connected to the backbone, a virtual link is used. This virtual
link will provide that area with a logical path to the backbone area. This virtual link is established
between two ABRs that are on one common area, with one of the ABRs connected to the backbone
area. In the example below two routers are connected to the same area (Area 1) but just one of them,
fw1, is connected physically to the backbone area.
Figure 4.2. Virtual Links Example 1
In the above example, the Virtual Link is configured between fw1 and fw2 on Area 1, as it is used as
the transit area. In this configuration only the Router ID has to be configured. The diagram shows
that fw2 needs to have a Virtual Link to fw1 with Router ID 192.168.1.1 and vice versa. These Vir-
tual Links need to be configured in Area 1.
Partitioned Backbone
OSPF allows for linking a partitioned backbone using a virtual link. The virtual link should be con-
figured between two separate ABRs that touch the backbone are from each side and having a com-
mon area in between.
83
Chapter 4. Routing