BGP Confederation Internal Connectivity Misconception
Short answer
B The statement that EBGP full-mesh connections are established inside each sub-AS is false, as internal sub-AS connectivity relies on IBGP, not EBGP.
Question
Key Takeaways
Confederations replace internal IBGP full-meshes with sub-AS structures linked by EBGP, but the internal links within those sub-ASs remain IBGP.
Identifies the false statement regarding BGP confederations: internal sub-AS links are IBGP, not EBGP. Learn why B is incorrect while confederations maintain a single AS view externally.
Common mistake: Many candidates assume that because sub-ASs use EBGP between them, all links involving sub-ASs must be EBGP. They overlook that the internal topology of a sub-AS still follows IBGP rules.
Expert Analysis
BGP Confederation Mechanics
BGP confederations are designed to reduce the IBGP full-mesh requirement within a large Autonomous System (AS). The AS is divided into multiple sub-ASs. Within each sub-AS, routers establish IBGP connections with each other (full-mesh) or use Route Reflectors. Between these sub-ASs, routers establish EBGP connections using special sub-AS numbers.
Analysis of Option B (The False Statement)
Option B states that "EBGP full-mesh connections are established in each sub-AS." This is false because the connections inside a sub-AS are IBGP, not EBGP. EBGP links exist only between sub-ASs. As noted in community comments, maintaining an IBGP full-mesh or using Route Reflectors is required within a sub-AS, making Option B the incorrect statement about confederation behavior.
Why Option A Sounds Plausible but is Correct
Option A claims that original IBGP attributes are retained. This is true because BGP Confederations use a modified version of EBGP for inter-sub-AS communication where the AS_PATH attribute is handled specially (sub-AS numbers are added but the original AS number is preserved in the confederation ID). To external peers, the entire confederation appears as a single AS.
External View and Configuration
Option D is correct: external peers see the confederation as one AS. Option C is technically correct in principle: the 'confederation ID' configured on routers represents the original public AS number, ensuring external consistency. Therefore, B is the only false statement.
Exam Strategy
When analyzing BGP confederation questions, strictly separate the scope of 'internal' (within a sub-AS) from 'external' (between sub-ASs). Remember that confederations only change how we handle the IBGP full-mesh problem; they do not convert internal IBGP sessions into EBGP sessions.
Frequently Asked Questions
Why do BGP confederations use sub-AS numbers?
Do external peers see the sub-AS numbers?
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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HCIP Resolutor 👍 1 Answered BLa afirmación que es verdaderamente FALSA en la teoría de BGP es la Opción B. Dentro de cada sub-AS (sub-autonomous system) creado dentro de una confederación, las conexiones que se establecen entre los routers son IBGP (no EBGP), y por lo tanto dentro de cada sub-AS se debe mantener la regla de Full-Mesh IBGP o usar Route Reflectors.
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