Click the exhibit. An update travels from AS 137000 to AS 135000, traversing both 32-bit and 16-bit AS Paths. What AS Path is seen by AS 200?

Question

Options

• AA. 140000 137000
• BB. 137000 140000
• CC. 23456 23456
• DD. 200 140000 137000

• A
  5% (2)
• B
  24% (10)
• C
  60% (25)
• D
  12% (5)

Explanation

Option C is correct because AS 200 is a legacy 2-byte (16-bit) AS speaker, and it cannot represent 4-byte AS numbers (those above 65535, like 137000 and 140000) in the AS_PATH attribute. Per RFC 6793, when a 4-byte AS number must be encoded for a 2-byte peer, it is replaced with the reserved placeholder AS 23456 (AS_TRANS) - one substitution per 4-byte AS hop, resulting in 23456 23456.

Why the distractors are wrong:

• A & B (140000 137000 / 137000 140000): A 16-bit AS speaker physically cannot store or advertise 4-byte AS numbers in the standard AS_PATH attribute - these values exceed the 16-bit maximum of 65535.
• D (200 140000 137000): AS 200 is the receiver of the update, not an originator prepending itself, and again, 4-byte ASNs can't appear in what a 2-byte speaker sees.

Memory tip: Think of 23456 as the "unknown stand-in" - whenever a 16-bit router encounters a 32-bit AS it can't name, it writes "23456" as a placeholder. Count the 32-bit hops in the path, and that's how many 23456s you'll see.

No community discussion yet for this question.