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246 CHAPTER 16: IP ROUTING PROTOCOL OPERATION
generated on the ABR, even though the default route 0.0.0.0 is not in the routing
table. On an ASBR, however, the default type-7 LSA route can be generated only if
the default route 0.0.0.0 is in the routing table.
Executing the
no-import-route command on the ASBR prevents the external routes
that OSPF imported through the
import-route command from advertising to the
NSSA. Generally, if an NSSA router is both ASBR and ABR, this argument is used.
The
default-cost command is used on the ABR attached to the NSSA. Using this
command, you can configure the default route cost on the ABR to NSSA.
By default, the NSSA is not configured, and the cost of the default route to the NSSA
is 1.
Configuring the Route Summarization of OSPF Area
Route summary means that ABR can aggregate information of routes of the same
prefix and advertise only one route to other areas. An area can be configured with
multiple aggregate segments allowing OSPF can summarize them. When the ABR
transmits routing information to other areas, it will generate Sum_net_Lsa (type-3
LSA) per network. If continuous networks exist in this area, you can use the
abr-summary command to summarize these segments into one segment. Thus, the
ABR only needs to send an aggregate LSA, and all the LSAs in the range of the
aggregate segment specified by the command are not transmitted separately.
Once the aggregate segment of a certain network is added to the area, all the
internal routes of the IP addresses in the range of the aggregate segment will no
longer be separately advertised to other areas. Only the route summary of the whole
aggregate network will be advertised. But if the range of the segment is restricted by
the parameter
not-advertise, the route summary of this segment is not advertised.
This segment is represented by IP address and mask.
Route summarization can take effect only when it is configured on ABRs.
Perform the following configuration in OSPF Area View.
By default, the inter-area routes will not be summarized.
Configuring Summarization of Imported Routes by OSPF
Perform the following configurations in OSPF View.
By default, summarization of imported routes is disabled.
Table 236 Configuring the route summarization of OSPF area
Operation Command
Configure the Route Summarization
of OSPF Area
abr-summary ip_address mask [ advertise |
not-advertise ]
Cancel the route summarization of
OSPF Area
undo abr-summary ip_address mask
Table 237 Configuring summarization of imported routes by OSPF
Operation Command
Configure summarization of
imported routes by OSPF
asbr-summary ip_address mask [ not-advertise |
tag value ]
Remove summarization of
routes imported into OSPF
undo asbr-summary ip_address mask