TP-Link JetStream T2700G-28TQ User Manual page 197

learn about the routing information from other areas, the size of the routing table of the routers in
the stub area as well as the number of the routing message transferred would be reduced greatly.
NSSA (Not-So-Stubby-Area) has a lot in common with stub area, but is not completely the same.
NSSA doesn't allow ABR to import the external routing information described by AS-External LSA,
either. But it does allow ASBR in the area to spread in the NSSA the routing information as Type-7
LSA, which is learned by other routing protocols. Upon receiving it, ABR in the area would
transform it to AS-External LSA and then flood to the whole autonomous system.
5. Route Summarization
Route summarization is to summarize routing information with the same prefix with a single
summarization route and then distribute it to other area. Via ABR route summarization a Summary
LSA will be distributed to other areas, while via ASBR route summarization an AS-External LSA
will be distributed to the entire AS. Therefore, route summarization will greatly reduce the size of
LSBD.
ABR Route Summarization: When the network reaches a certain size, to configure route
summarization on the ABR could summarize the intra-area route to be a wider one and then
distribute it to other areas, which could receive less the routing entries. As Figure 10-37 shows, in
Area 1 ABR1 can configure a summarization route 192.161.0.0/16 and advertise it to the
backbone area, while in Area 2 ABR2 can configure an summarization route 192.162.0.0/16 and
advertise it to the backbone area.
Please pay attention to that, if the network is planned to be discontinuous subnets, you need to
configure the route summarization with great caution; otherwise, it might cause some unreachable
network conditions. As Figure 10-38 shown, configuring the summarization route 192.161.0.0/16
on ABR1 and ABR2 might result in the inaccessible routing. Under such circumstance, it is
suggested to configure route summarization on only one ABR.
ASBR Route Summarization: If a route summarization is configured on an ASBR, the
AS-External LSA in the specified address range will be summarized. When NSSA is configured,
Type-7 LSA in the specified address range will also be summarized. Following a similar principle
with ABR route summarization, ASBR summarizes routes of different type.
Figure 10-37 ABR Route Summarization
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