300-320 Question #660: Real Exam Question with Answer & Explanation

The correct answer is D: The AS uses multiple sub-ASs to scale.. D is correct: The defining characteristic of BGP confederations is that a single large AS is divided into multiple smaller sub-ASes (using private AS numbers internally). Each sub-AS runs IBGP internally among its members, and uses EBGP-like sessions between sub-ASes. This divisi

Question

Which statements are true about confederations? (Select two.)

Options

AConfederations can be used with any routing protocol.
BConfederations modify the split-horizon rule.
CConfederations are IBGP speakers that reflect learned routes from IBGP peers to other IBGP
DThe AS uses multiple sub-ASs to scale.
EConfederations add more information to the AS path.

Explanation

D is correct: The defining characteristic of BGP confederations is that a single large AS is divided into multiple smaller sub-ASes (using private AS numbers internally). Each sub-AS runs IBGP internally among its members, and uses EBGP-like sessions between sub-ASes. This division into sub-ASes is exactly how confederations achieve scalability. E is correct: As routes traverse confederation sub-AS boundaries, the sub-AS numbers are prepended to a special confederation AS_PATH attribute. This adds path information that reflects the sub-AS hops taken within the confederation. This extra path data is used for loop prevention within the confederation but is stripped before routes are advertised to external (non-confederation) BGP peers. A is false because confederations are a BGP-specific feature - not applicable to any routing protocol. B is false because modifying the split-horizon rule describes route reflectors, not confederations. C is false because reflecting routes between IBGP peers describes route reflectors, not confederations - confederations use sub-AS segmentation with inter-sub-AS EBGP-like sessions.

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