Extreme Networks ExtremeWare XOS 10.1, Руководство пользователя

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Overview of OSPF
ExtremeWare XOS 10.1 Concepts Guide 191
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External routes originating from the NSSA can be propagated to other areas, including the backbone
area.
The CLI command to control the NSSA function is similar to the command used for configuring a stub
area, as follows:
configure ospf area <area-identifier> nssa [summary | nosummary] stub-default-cost
<cost> {translate}
The
translate
option determines whether type 7 LSAs are translated into type 5 LSAs. When
configuring an OSPF area as an NSSA, the
translate
should only be used on NSSA border routers,
where translation is to be enforced. If
translate
is not used on any NSSA border router in a NSSA, one
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 33 illustrates a virtual link.
NOTE
Virtual links cannot be configured through a stub or NSSA area.