When an Alcatel-Lucent 7750 SR receives a BGP route with an AS Path loop, what action is taken by default?

Question

Options

• AThe route is ignored.
• BThe route is flagged as invalid and placed in the LOC-RIB.
• CThe route is flagged as invalid and placed in the RIB-In.
• DThe route is flagged as invalid and placed in the RIB-Out.

• A
  4% (2)
• B
  15% (8)
• C
  75% (40)
• D
  6% (3)

Explanation

When the 7750 SR detects its own AS number in the received AS_PATH, it flags the route as invalid due to loop prevention, but still stores it in the RIB-In - the inbound table where all received BGP prefixes land before policy evaluation and best-path selection. Option C is correct because the route is retained (not silently dropped) and marked invalid at this earliest stage, never advancing further in the RIB pipeline.

Why the distractors are wrong:

• A is wrong because the route is not silently discarded - it is kept in RIB-In with an invalid flag, which allows operators to inspect it.
• B is wrong because the LOC-RIB holds only valid, policy-accepted routes from which the best path is chosen; an invalid route never reaches it.
• D is wrong because RIB-Out holds routes prepared for advertisement to peers - an invalid route would never progress that far downstream.

Memory tip: Think of the BGP RIB pipeline as a three-stage conveyor belt: RIB-In → LOC-RIB → RIB-Out. A loop-detected route gets a red "invalid" sticker at the very first stage (RIB-In) and never moves down the belt.

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