3Com Switch 7700 Configuration Manual page 124

116 C 5: IP R
HAPTER OUTING
P O
ROTOCOL PERATION
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 the routes of the
same prefix and advertise only one route to other areas. An area can be
configured with multiple aggregate segments allowing OSPF to summarize them.
When the ABR transmits routing information to other areas, it generates
Sum_net_Lsa (type-3 LSA) per network. If some 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. Therefore, the sizes of the LSDBs in other areas can be
reduced.
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 are no
longer separately broadcast to other areas. Only the route summary of the whole
aggregate network is advertised. However, if the range of the segment is restricted
by the not-advertise keyword, the route summary of this segment is not
advertised. This segment is represented by an IP address and mask. The receiving
and restriction of the aggregate segment can reduce the routing traffic exchanged
between the areas.
Route summarization can take effect only when it is configured on ABRs.
Perform the following configuration in OSPF Area view.
Table 38 Configuring the Route Summarization of an OSPF Area
Operation
Configure the Route Summarization of OSPF
Area
Cancel route summarization of OSPF Area
By default, the inter-area routes are not summarized.
Configuring OSPF Virtual Link
According to RFC2328, after the area division of OSPF, the backbone is established
with an area-id of 0.0.0.0. The OSPF routes between non-backbone areas are
updated with the help of the backbone area. OSPF stipulates that all the
non-backbone areas should maintain connectivity with the backbone area, and at
least one interface on the ABR should fall into the area 0.0.0.0. If an area does not
Command
abr-summary ip-address mask [ advertise |
not-advertise ]
undo abr-summary ip-address mask