Assuming a full iBGP mesh in ISP 1, how does router R1 in ISP 1 handle a BGP update received from ISP 2?

Question

Options

• AThe update is sent to router R2 in ISP 1.
• BThe update is sent to routers R2 and R3 in ISP 1.
• CThe update is sent to all routers in ISP 1.
• DThe update is sent to routers R2 and R3 in ISP 1, and to router R3 in ISP 2.

• A
  11% (5)
• B
  2% (1)
• C
  80% (36)
• D
  7% (3)

Explanation

In a full iBGP mesh, every router within an AS has a direct iBGP session with every other router in that AS - so when R1 receives an eBGP update from ISP 2, it must advertise that route to all iBGP peers inside ISP 1, making C correct. This is the fundamental purpose of the full mesh: ensuring complete route visibility across the AS without relying on route reflectors or confederations.

A is wrong because limiting propagation to only R2 would leave other routers without the external route - a full mesh requires complete distribution. B is wrong for the same reason; even if R2 and R3 are the only named routers in the question, the principle is "all routers," and partial distribution defeats the mesh. D is wrong because R1 would never send an iBGP-distributed route back to ISP 2 - that would create a routing loop, and the update already originated from ISP 2's eBGP session.

Memory tip: Think "eBGP in → iBGP everywhere." A route arriving via eBGP (from outside the AS) triggers full internal flooding across the iBGP mesh, while a route learned via iBGP is not re-advertised to other iBGP peers (split-horizon rule) - but that rule doesn't apply here since the source is external.

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