3Com Switch 4800G 24-Port Configuration Manual page 280

280 C 28: OSPF C
HAPTER
Classification of OSPF
Networks
ONFIGURATION
OSPF has two types of route summarization:
1 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.
2 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 prioritize routes into four levels:
Intra-area route
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Inter-area route
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Type-1 external route
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Type-2 external route
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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: Type-1 and Type-2. A Type-1
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 Type-1 external route= the cost
from the router to the corresponding ASBR+ the cost from the ASBR to the
destination of the external route.
A Type-2 external route is an EGP route, which has low credibility, so OSPF
considers the cost from the ASBR to the destination of the Type-2 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 Type-2 external route=
the cost from the ASBR to the destination of the Type-2 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 types
OSPF classifies networks into four types upon the link layer protocol:
Broadcast: When the link layer protocol is Ethernet or FDDI, OSPF considers the
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network type broadcast by default. On Broadcast networks, packets are sent to
multicast addresses (such as 224.0.0.5 and 224.0.0.6).