Not-So-Stubby Areas (NSSAs) 229
Totally Stubby Area
A totally stubby area allows only the default summary link to be propagated into the area by the
ABR. No redistribution can occur, and no other routing protocols can exist. No Type 3 and Type
5 LSAs are flooded into a totally stubby area. Only a single default route is sent.
NOTE
Not all vendors support the totally stubby feature. When including totally stubby as a design
concept, be sure that it is supported if you expect your OSPF-area network to contain routers
from other vendors.
Not-So-Stubby Areas (NSSAs)
Support for not-so-stubby areas (NSSAs) is described in RFC 1587. NSSAs allow you to treat
complex networks similar to stub areas. This can simplify your network topology and reduce
OSPF-related traffic.
NSSAs and Type 7 LSAs
NSSAs are similar to stub areas, except that they allow limited importing of autonomous system
(AS) external routes. External routes carried into the backbone are not propagated into the area.
Intra-area routes are propagated into the NSSA. NSSA is an area with an ASBR that allows
external Type 7 LSAs into the OSPF network while retaining the characteristics of a stub area.
The Type 7 LSA is generated by the ASBR in the NSSA. Type 7 LSAs are similar to Type 5
LSAs, except for the following:
·
NSSAs can originate and import Type 7 LSAs. Like stub areas, NSSAs cannot originate
or import Type 5 LSAs.
·
Type 7 LSAs can be advertised only within a single NSSA. They are not flooded
throughout the AS, as are Type 5 LSAs.
External Link Advertisement
OSPF classifies external routes into two types:
·
External Type 1 routes use a metric that is the sum of the external metric and the collective
internal cost of reaching the destination.
·
External Type 2 routes use a metric that examines the external metric and does not take
the internal cost into consideration.
The E1 metric is the sum of the internal and external metrics. The E2 metric does not add
internal metrics.
87200333.book Page 229 Wednesday, August 22, 2001 2:37 PM