Regarding BGP route announcement, which of the following descriptions is wrong? (Multiple choice)

Question

Options

• AOnly routes learned from IGP can be advertised to other BGP neighbors.
• BAll BGP routes learned from all BGP neighbors are advertised to other BGP neighbors.
• COnly the best route preferred by BGP can be announced to other BGP neighbors.
• DBy default, routes learned from IBGP are not forwarded to other IBGP neighbors.

• A
  81% (26)
• B
  3% (1)
• C
  6% (2)
• D
  9% (3)

Explanation

A is the wrong description because BGP is not restricted to advertising only IGP-learned routes - it can redistribute and announce routes from static routes, directly connected networks, and routes learned from other BGP sessions as well. B is actually also a wrong description (the stated correct answer may be incomplete): due to IBGP split-horizon, routes learned from an IBGP peer are not re-advertised to other IBGP peers, so "all routes from all neighbors" is false - but since D correctly captures this rule, B and D contradict each other, making B the implicit trap. C is a correct statement: BGP performs best-path selection and only installs and advertises a single best route per prefix to its neighbors, which is standard BGP behavior. D is also correct: the IBGP full-mesh requirement exists precisely because IBGP split-horizon prevents route propagation between IBGP peers - this is why network engineers deploy route reflectors or confederations.

Memory tip: Think of BGP as "best one out, not IGP-only in, and never loop IBGP back." - it sends only its best path (C), accepts routes from any source not just IGP (A is wrong), and blocks IBGP-to-IBGP re-advertisement to prevent loops (D).

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