Information Technology Reference
In-Depth Information
Ta b l e 1 1- 4
Major LSA Types
Type
Code
Type
Description
5
Autonomous system ex-
ternal LSA
Originated by ASBRs. Advertises destinations external
to the OSPF autonomous system, flooded throughout
the whole OSPF autonomous system.
7
Not-so-stubby area
(NSSA) external LSA
Originated by ASBRs in an NSSA. It is not flooded
throughout the OSPF autonomous system, only to the
NSSA. Similar to the Type 5 LSA.
Ty pe 1 and Ty pe 2 L SAs are intra- area L SAs that haven an area-flooding scope. Ty pe 3
LSAs area a summary of destinations outside the local area but within the OSPF domain.
Ty pe 4 L SAs prov ide reachabilit y about the ASBR. Ty pe 3 and Ty pe 4 L SAs are interarea
LSAs that have an area-flooding scope. ABRs exchange Type 3 and Type 4 LSAs. Type 5
LSAs advertise external destinations. Type 5 LSAs have a domain-flooding scope, mean-
ing they are flooded throughout all areas.
Autonomous System External Path Types
The two types of autonomous system external paths are Type 1 (E1) and Type 2 (E2), and
they are associated with Type 5 LSAs. ASBRs advertise external destinations whose cost
can be just a redistribution metric (E2) or a redistribution metric plus the costs of each
segment (E1) used to reach the ASBR.
By default, external routes are of Type 2, which is the metric (cost) used in the redistribu-
tion. Type 1 external routes have a metric that is the sum of the redistribution cost plus
the cost of the path to reach the ASBR.
OSPF Stub Area Types
OSPF provides support for stub areas. The concept is to reduce the number of interarea or
external LSAs that get flooded into a stub area. RFC 2328 defines OSPF stub areas. RFC
1587 defines support for NSSAs. Cisco routers use totally stubby areas, such as Area 2, as
shown in Figure 11-5.
Stub Areas
Consider Area 1 in Figure 11-5. Its only path to the external networks is via the ABR
through Area 0. All external routes are flooded to all areas in the OSPF autonomous sys-
tem. You can configure an area as a stub area to prevent OSPF external LSAs (Type 5)
from being flooded into that area. A single default route is injected into the stub area in-
stead. If multiple ABRs exist in a stub area, all inject the default route. Traffic originating
within the stub area routes to the closest ABR.
Note that network summary LSAs (Type 3) from other areas are still flooded into the
stub Area 1.
 
 
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