BGP confederations are widely used because they can reduce the number of IBGP connections in...

Comprehensive and Detailed in-depth Step-by-Step Explanation:

BGP confederations are used to reduce the complexity of IBGP full-mesh connections within a large AS by dividing the AS into multiple sub-ASs (called "member ASs") while still appearing as a single AS to external ASs. Below is an analysis of each option:

A. The ASs outside a confederation still consider the confederation as one AS:

This istrue.Externally, the entire confederation is seen as a single AS, identified by the main AS number.

B. After a confederation is configured, the original IBGP attributes can be retained:

This istrue.The configuration of a confederation does not affect the IBGP attributes exchanged between routers.

C. After a confederation is configured, the original AS number is used as the confederation ID of each router:

This istrue.The main AS number is used as the Confederation Identifier for all member ASs in the confederation.

D. A confederation divides an AS into several sub-ASs, and EBGP full-mesh connections are established in each sub-AS:

This isfalse.Within each sub-AS of a confederation,IBGP connectionsare still used (not EBGP). Confederations are designed to reduce the number of IBGP full-mesh connections required within a large AS by creating smaller, manageable sub-ASs.

Therefore, the correct answer isD.