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Recall that summarization is the
consolidation of multiple routes into one single, supernet
advertisement (see Chapter 2 for more details). Proper summarization
requires contiguous (sequential) addressing (for example,
200.10.0.0, 200.10.1.0, 200.10.2.0, and so on). OSPF routers can be
manually configured to advertise a supernet route, which is
different from an LSA summary route.
Route summarization directly affects
the amount of bandwidth, CPU, and memory resources that are consumed
by the OSPF process. With summarization, if a network link fails or
flaps, the topology change will not be propagated into the backbone
(and other areas by way of the backbone). As discussed in previous
chapters, route summarization protects routers from needless routing
table recalculations. Because the SPF calculation places a
significant demand on the router's CPU, proper summarization is an
imperative part of OSPF configuration.
OSPF supports two types of
summarization:
- Interarea route summarization
- Interarea route summarization is done on ABRs and applies to
routes from within each area. It does not apply to external
routes injected into OSPF via redistribution. To take advantage
of summarization, network numbers within areas should be
contiguous.
- External route summarization
- External route summarization is specific to external routes
that are injected into OSPF via redistribution. Here again, it
is important to ensure that external address ranges that are
being summarized are contiguous. Summarization of overlapping
ranges from two different routers could cause packets to be sent
to the wrong destination. Only ASBRs can summarize external
routes.
To configure an ABR to summarize
routes for a specific area before injecting them into a different
area, you use the following syntax:
Router(config-router)# area
area-id range address mask
To configure an ASBR to summarize
external routes before injecting them into the OSPF domain, you use
the following syntax:
Router(config-router)# summary-address
address mask
To configure RTA in the figure for
external route summarization, you can use the following commands:
RTA(config)# router ospf 1
RTA(config-router)# summary-address 200.9.0.0 255.255.0.0
Once configured, RTA will send only a
single summary route, 200.9.0.0/16, into the OSPF domain.
Because RTB sits on the border
between Area 0 and Area 1, it should be configured to perform
interarea summarization, as shown:
RTB(config)# router ospf 1
RTB(config-router)# area 1 range 192.168.16.0 255.255.252.0
Note that the area 1 range
command in this example specifies the area containing the range to
be summarized before being injected into Area 0.
Also, note that, depending on your
network topology, you may not want to summarize area 0 networks. If
you have more than one ABR between an area and the backbone area,
for example, sending a summary LSA with the explicit network
information will ensure that the shortest path is selected. If you
summarize the addresses, a suboptimal path selection may occur.
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