Which regular expression will match the AS Path of a route originating in remote AS 65100?

Question

Options

• A".* 65100"
• B"65100+"
• C".*65100$"
• D"_65100"

• A
  79% (22)
• B
  11% (3)
• C
  4% (1)
• D
  7% (2)

Explanation

Option A (".* 65100") correctly matches a route originating in AS 65100 because, in BGP, the originating AS always appears last (rightmost) in the AS path - transit ASes prepend themselves to the left as the route propagates. The .* matches any sequence of preceding transit AS numbers, and 65100 (space + AS number) anchors the match to the final position, confirming AS 65100 as the origin.

B ("_65100_+") is wrong because the trailing _+ requires one or more delimiter characters after 65100, which means 65100 cannot be the last entry - this would only match 65100 as a transit AS, not the originator.

C (".*_65100_$") is wrong because the _ before $ demands an additional delimiter character after 65100, but nothing follows the originating AS at the end of the path, making the pattern either impossible to satisfy or logically redundant.

D ("_65100") is wrong because it lacks an end anchor, so it matches AS 65100 anywhere in the path - including when it's merely a transit AS - without confirming it's the origin.

Memory tip: "The originator is always last - match anything before it with .*, then the space and AS number at the tail."

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