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, the topology change
will not be propagated into the backbone (and other areas by way of
the backbone). As such, flooding outside the area will not occur, so
routers outside the area with the topology change will not have to
run the SPF algorithm (comment – this is explained in the
previous chapter. Running the SPF algorithm is a CPU-intensive
activity.
Following are the two types of summarization:
- Inter-area route summarization
--- Inter-area 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. In order to take advantage of summarization,
network numbers within areas should be assigned in a contiguous
way so as to be able to consolidate these addresses into one
range. The main figure illustrates where
inter-area summarization occurs.
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 overlapping ranges from two different routers could
cause packets to be sent to the wrong destination. Only ASBRs can
summarize external routes. These types of routes cannot be
summarized by any other router type than an ASBR.
In order to take advantage of summarization, network numbers in
areas should be assigned in a contiguous way to be able to group
these addresses into one range.
In the main Figure, for example, the list
of six networks in router B's routing table can be summarized into
two summary address advertisements.