As shown in the figure, RE_D calls ip as-path-filter 1 in the inbound direction, then the route 10.0.0.0/24 will be learned by RE_D from the path (___).

Question

Options

• A00
• B
• C
• D

• A
  75% (24)
• B
  3% (1)
• C
  16% (5)
• D
  6% (2)

Explanation

Option A is correct because the ip as-path-filter 1 applied inbound on RE_D permits only BGP routes whose AS_PATH attribute matches the regular expression defined in filter 1, and the route 10.0.0.0/24 arriving via the path represented by "00" is the only one that satisfies that match condition. An AS-path filter works like an access list for the BGP AS_PATH attribute: routes whose paths do not match are dropped before being installed in the routing table, so only the permitted path survives. The other options (B, C, D) represent alternative AS paths through which the same prefix might arrive, but their AS_PATH strings do not match filter 1, so those advertisements are rejected at the inbound policy check on RE_D.

Memory tip: Think of ip as-path-filter as a bouncer checking ID at the door - only BGP routes carrying the AS path that matches the filter's regular expression are let in; all others are turned away before they ever reach the routing table.

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