6.5 Configuring EIGRP
6.5.6 Summarizing EIGRP routes for IP
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