Allied Telesis AT-8550 - Manual (Page 161)

AT-9108, AT-8518, AT-8525, and AT-8550 User’s Guide
9-7
Not-So-Stubby-Areas (NSSA). NSSAs are similar to the existing
OSPF stub area configuration option, but have the following two
additional capabilities:
❑ External routes originating from an ASBR connected to the
NSSA can be advertised within the NSSA.
❑ 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:
config ospf area nssa {summary |
nosummary} stub-default-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 any of the
following:
❑ 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 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 9-1 illustrates a virtual link.