Ospf Network Type - H3C S5600 SERIES Operation Manual

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Figure 4-6 Route summarization

OSPF has two types of route summarization:

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.

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

OSPF prioritizes routes into four levels:

Intra-area route

Inter-area route

type1 external route

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

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:

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