Click the exhibit. An update travels from AS 135000 to AS 137000, traversing both 32-bit and 16-bit AS paths. What will the AS Path be when the update reaches AS 137000?

Question

Options

• A140000 200 135000
• B137000 140000 200 135000
• C140000 23456 135000
• D135000 200 140000

• A
  72% (44)
• B
  5% (3)
• C
  8% (5)
• D
  15% (9)

Explanation

Option A is correct because when a BGP update carrying a 32-bit AS number (135000) traverses a 16-bit-only AS (AS 200), the 32-bit ASN is temporarily replaced with AS_TRANS (23456) in the AS_PATH. However, the true 32-bit path is preserved in the AS4_PATH attribute. When the next 32-bit-capable router (AS 140000) receives the update, it merges the AS_PATH and AS4_PATH to reconstruct the correct path, substituting 23456 back with 135000. AS 140000 then prepends its own ASN before forwarding to AS 137000, yielding 140000 200 135000.

Why the distractors fail:

• B incorrectly includes AS 137000 itself - a router never sees its own ASN prepended to an incoming update; that only happens when it advertises the route onward.
• C (140000 23456 135000) would be correct only if AS 140000 were a 16-bit-only router that could not process AS4_PATH and therefore left the AS_TRANS placeholder in place - but AS 140000 is 32-bit capable and performs the merge.
• D has the AS_PATH in the wrong direction; BGP AS_PATH is prepended left-to-right as the update travels, so the origin (135000) is always rightmost.

Memory tip: Think of AS_TRANS (23456) as a "claim ticket" - the 32-bit ASN is checked in at the 16-bit boundary, and the next 32-bit-capable router redeems the ticket to restore the real ASN. If no 32-bit router picks it up, the 23456 stays.

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