EIGRP automatically summarizes routes at the
classful boundary (that is, the boundary where the network address ends
as defined by class-based addressing). This means that even though RTC
is connected only to the subnet 2.1.1.0, it will advertise that it is
connected to the entire Class A network, 2.0.0.0. In most cases, auto
summarization is a good thing; it keeps routing tables as compact as
possible (see Figure ).
However, as we saw in Chapter 2, IP
Addressing, you may not want automatic summarization to occur. If
you have discontiguous subnetworks, as shown in Figure ,
auto-summarization must be disabled for routing to work properly. To
turn off auto-summarization, use the following command:
router(config-router)#no
auto-summary
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