TP-Link T3700G-52TQ User Manual page 223

the Type-2 external route equals to that 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.
Intra-area route and inter-area route describe the internal network structure of the
autonomous system, while the external routes tell how to select the route to the destination
outside the autonomous system.
Stub Area and NSSA Area
4.
An area that can connect to the autonomous system and forward the communication data to
external areas only through ABR could be set as Stub Area. Once an area is set to be Stub Area,
ABR would no longer flood the external routing information described by the AS-External LSA
to it, and meanwhile a default route with a target network 0.0.0.0 would be generated. This
default routing would be announced to the other routers in the area. All the packets forwarded
to external areas would be sent to ABR and then be forwarded outwards through it. Since there
is no need to 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.
Route Summarization
5.
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.
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