The original path of a BGP route is (100 200 300). After using the apply as-path 400 overwrite command in conjunction with the routing policy, the AS_Path of the route becomes which of the following?

Question

Options

• A(100)
• B(400 100 200 300)
• C(100 200 300 400)
• D(100 400 200 300)

• A
  75% (27)
• B
  6% (2)
• C
  6% (2)
• D
  14% (5)

Explanation

Option A is correct because the overwrite keyword in apply as-path completely replaces the existing AS_PATH using the referenced AS-path filter (filter 400 in this case), which is configured to match and remove the transit AS entries 200 and 300 - leaving only the originating AS (100) intact. Options B, C, and D are all wrong for the same underlying reason: each one shows the number 400 being preserved somewhere in the path (prepended at the front in B, appended at the end in C, or inserted in the middle in D), which reflects additive/prepend behavior, not overwrite - the overwrite action replaces path entries rather than inserting alongside them. Note that 400 here is an AS-path filter number, not a new AS number being written into the path, which is why the result contains no AS 400 at all.

Memory tip: Think of overwrite like a find-and-replace in a text editor - the filter (400) finds the matching AS entries and the overwrite replaces (removes) them, leaving only what wasn't matched. If the keyword were additive, you'd see 400 added to the path, which is exactly what the wrong answers all show.

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