When designing a network .Which method can be used to control the exit point for traffic an autonomous system, at the layer 3 control plane?

Question

Options

• APrepending AS path.
• BTuning the multi-exit discriminator.
• CSetting the site of Origin extended community.
• DTuning the metric of the under-tying IGP.

• A
  6% (3)
• B
  11% (6)
• C
  22% (12)
• D
  61% (33)

Explanation

Tuning the underlying IGP metric is correct because the IGP (e.g., OSPF cost, IS-IS metric) governs how internal routers select paths to reach BGP next-hops. By raising or lowering the metric toward a specific border router, you steer traffic toward your preferred exit point - a pure layer 3 control plane action operating entirely within the autonomous system.

Why the distractors are wrong:

• A (AS path prepending): This manipulates outbound BGP advertisements to make your AS look less attractive to external peers, influencing inbound traffic from other ASes, not your own exit selection.
• B (MED): MED is advertised to a neighboring AS to suggest which of your entry points it should use to send traffic into you - it controls ingress, not egress.
• C (Site of Origin): SoO is an MPLS/VPN extended community used to prevent routing loops in hub-and-spoke topologies; it has no role in general exit-point selection.

Memory tip: Think "I Go Pick the door" - the IGP metric is your internal knob for picking which exit door (border router) traffic walks out of. Anything BGP-based (MED, AS-path) is about signaling outside your AS, so it can't be the answer when the question is about your own exit control.

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