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.
[References:, Huawei HCIA-Datacom Study Guide, Chapter on "BGP Confederations"., RFC 5065 – Autonomous System Confederations for BGP., , , ]
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