Which of the following best describes the purpose of advertise-external?

Question

Options

• AIt enables a BGP speaker to advertise multiple external routes to iBGP peers.
• BIt enables a BGP speaker to advertise its best external route to iBGP peers when the best overall route is an internal.
• CIt enables a BGP speaker to advertise its best external route to eBGP peers without an import policy.
• DIt enables a BGP speaker to advertise its best internal route to eBGP peers without an export policy.

• A
  13% (2)
• B
  80% (12)
• D
  7% (1)

Explanation

Option B is correct because advertise-external solves a specific iBGP limitation: normally, if a router's best path for a prefix is an iBGP-learned (internal) route, it will not advertise any competing eBGP route to its iBGP peers. With advertise-external enabled, the router will additionally send its best eBGP-learned route to iBGP peers alongside the best internal route, giving peers more path options for redundancy and load-balancing.

Why the others are wrong:

• A is wrong because advertise-external advertises the single best external route - not multiple external routes. (That's a different feature, such as add-path.)
• C is wrong on two counts: the target peers are iBGP, not eBGP, and import policy is unrelated to this feature.
• D is wrong because the feature advertises the best external (eBGP-learned) route, not an internal one - and again, the target is iBGP peers, not eBGP peers.

Memory tip: Focus on the word "external" in the feature name - it's about getting an external path seen by internal (iBGP) peers. When your best overall route is already internal, advertise-external lets you say: "Hey iBGP peers, here's also what I learned from outside, just in case."

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