Unlike EIGRP, OSPF does not automatically summarize at Classful boundaries. In addition to this, OSPF summarization is different for internal and external routes, which often results in confusions and misconfigurations. In Cisco IOS software, internal route summarization is configured by using the area [area ID] range [<address> <mask> [advertise | not-advertise] [cost <cost>] router configuration command on the Area Border Router (ABR). Following the configuration of the summary on the ABR, the following occurs:
- An intra-area route for the summary pointing to Null0 is installed into the routing table . . .
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