Extreme Networks Summit Summit24 Installation And User Manual page 189

Overview of OSPF
where translation is to be enforced. If is not used on any NSSA border router in a NSSA, one
translate
of the ABRs for that NSSA is elected to perform translation (as indicated in the NSSA specification). The
option should not be used on NSSA internal routers. Doing so inhibits correct operation of the election
algorithm.
Normal Area
A normal area is an area that is not:
• Area 0.
• Stub area.
• NSSA.
Virtual links can be configured through normal areas. External routes can be distributed into normal
areas.
Virtual Links
In the situation when a new area is introduced that does not 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 normal area that connects to the backbone. A virtual link must be
established between two ABRs that have a common area, with one ABR connected to the backbone.
Figure 30 illustrates a virtual link.
NOTE
Virtual links can not be configured through a stub or NSSA area.
Figure 30: Virtual link using Area 1 as a transit area
Virtual link
ABR ABR
Area 2 Area 1 Area 0
EW_016
Virtual links are also used to repair a discontiguous backbone area. For example, in Figure 31, 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.
Summit24e3 Switch Installation and User Guide 189