Extreme Networks ExtremeWare XOS 11.3, Manual

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OSPF
ExtremeWare XOS 11.3 Concepts Guide
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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 68 illustrates a virtual link.
NOTE
Virtual links cannot be configured through a stub or NSSA area.
Figure 68: Virtual link using area 1 as a transit area
Virtual links are also used to repair a discontiguous backbone area. For example, in Figure 69 , 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.
ABR ABR
EX_044
Area 2 Area 1 Area 0
Virtual link