Click on the exhibit. AS 65540 is using MPLS to provide BGP shortcuts for its iBGP routing. AS 65541 is advertising prefix 192.168.1.0/27 into BGP. Which routers have the BGP update for the…

Question

Options

• ARouters R1, R2, R3, R4 and R5.
• BRouters R1 and R5 only.
• CRouters R1, R2, R3 and R4 only.
• DRouters R1, R4 and R5 only.

• A
  10% (2)
• B
  33% (7)
• C
  14% (3)
• D
  43% (9)

Explanation

Option D is correct because in AS 65540, only the PE (Provider Edge) routers - R1, R4, and R5 - participate in BGP. When MPLS is used to provide BGP shortcuts, the core P (Provider) routers R2 and R3 forward traffic purely via MPLS label switching and do not run BGP, so the prefix 192.168.1.0/27 never appears in their BGP tables.

• A is wrong because it includes R2 and R3, which are core P routers - MPLS shortcuts exist specifically so these intermediate routers don't need BGP.
• B is wrong because it omits R4, which is also a PE router with an active iBGP session and therefore receives the update.
• C is wrong on two counts: it incorrectly includes R2 and R3 (P routers, no BGP), and it excludes R5 (a PE router that does have BGP).

Memory tip: Use the phrase "PE routers Play BGP, P routers just Push Labels." Only Provider Edge routers maintain BGP tables; the core Provider routers rely on MPLS label forwarding - that's the entire point of BGP shortcuts over MPLS.

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