Regarding the range of BGP AS, which of the following descriptions are incorrect? (Multiple choice)

Question

Options

• AWhen 2-byte AS is supported, the value range of the private AS number is 32768-65535
• BWhen 2-byte AS is supported, the value range of the private AS number is 64512-65535
• CWhen supporting 2-byte AS, the value range is 0-65535.
• DWhen 4-byte AS is supported, no new attributes need to be defined

• A
  79% (42)
• B
  21% (11)

Explanation

Options A, C, and D are the incorrect descriptions of BGP AS ranges:

• A is wrong because it misquotes the private AS boundary. The private 2-byte AS range is 64512–65535 (per RFC 1930/6996), not 32768–65535. The 32768 threshold has no BGP standards basis.
• C is wrong because AS number 0 is reserved and not a valid AS number; the usable 2-byte range begins at 1, not 0. Stating "0–65535" overstates the valid range.
• D is wrong because 4-byte AS support (RFC 6793) did require two new optional transitive attributes - AS4_PATH and AS4_AGGREGATOR - to maintain backward compatibility with legacy 2-byte-only peers.

B is correct (and thus not in the answer): 64512–65535 is the accurately defined private AS range for 2-byte ASNs.

Memory tip: Associate private 2-byte ASNs with "64K starts it" - private space begins just above 64,000 (at 64512). If you see 32768, that's a distractor borrowing from IPv4 private-range thinking, not BGP.

No community discussion yet for this question.