H3C Technologies H3C S3600 Series Switches - Manual (Page 335)

4-8
Figure 4-6 Route summarization
Router A
ABR
Router B
ABR Area 0
19.1.1.0/24
19.1.2.0/24
19.1.3.0/24
……
Area 1
19.1.0.0/16
OSPF has two types of route summarization:
2) ABR route summarization
To distribute routing information to other areas, an ABR generates Type-3 LSAs on a per network
segment basis for an attached non-backbone area. If contiguous network segments are available in the
area, you can summarize them with a single network segment. The ABR in the area distributes only the
summary LSA to reduce the scale of LSDBs on routers in other areas.
3) ASBR route summarization
If summarization for redistributed routes is configured on an ASBR, it will summarize redistributed
Type-5 LSAs that fall into the specified address range. If in an NSSA area, it also summarizes Type-7
LSAs that fall into the specified address range.
If this feature is configured on an ABR, the ABR will summarize Type-5 LSAs translated from Type-7
LSAs.
Route types
OSPF prioritizes routes into four levels:
z Intra-area route
z Inter-area route
z type1 external route
z type2 external route
The intra-area and inter-area routes describe the network topology of the AS, while external routes
describe routes to destinations outside the AS. OSPF classifies external routes into two types: type1
and type2.
A type1 external route is an IGP route, such as a RIP or static route, which has high credibility and
whose cost is comparable with the cost of an OSPF internal route. The cost from a router to the
destination of the type1 external route= the cost from the router to the corresponding ASBR+ the cost
from the ASBR to the destination of the external route.
A type2 external route is an EGP route, which has low credibility, so OSPF considers the cost from the
ASBR to the destination of the type2 external route is much bigger than the cost from the ASBR to an
OSPF internal router. Therefore, the cost from the internal router to the destination of the type2 external
route = the cost from the ASBR to the destination of the type2 external route. If two routes to the same
destination have the same cost, then take the cost from the router to the ASBR into consideration.
OSPF Network Type
Four OSPF network types
OSPF divides networks into four types by link layer protocols: