Which configuration summarizes external routes in the 172.16.0.0/22 range into a single prefix on an ABR for Area 1?

The correct choice is based on the configuration that allows for summarization of external routes in OSPF, particularly in a not-so-stable area such as a Not-So-Stubby Area (NSSA). In this case, the configuration directly specifies an area-range, which facilitates the summarization of external routes into a single prefix for the designated area.

Using area-range under NSSA indicates that external routes within the specified range (172.16.0.0/22) are aggregated into one summarized route. This is particularly useful for reducing the amount of routing information that needs to be processed and stored by routers within the area. When summarization is appropriately configured, it allows routers to maintain a more efficient routing table with fewer entries, thereby optimizing routing performance in the network.

The other options fail to accomplish this goal in the following ways:

• One option indicates a configuration for the NSSA with a default metric, which does not directly pertain to summarizing routes in the way the question specifies.

• Another option refers to using stub summarization; however, this is not applicable to the NSSA context or specifically summarizing external routes.

• The last option mentions restricting area range in a NSSA but does not conform to the required configuration