hardMultiple Choice
300-410 BGP is used between two ISPs Practice Question
BGP is used between two ISPs. Router R1 has: neighbor 10.0.0.2 route-map SET-MED in, route-map SET-MED permit 10, set metric 50. Router R2 shows: show ip bgp 172.16.0.0 includes MED 50 but the path is not preferred. What is the root cause?
⚠ Common exam trap
Cisco often tests the nuance that MED is only compared between paths from the same neighboring AS, leading candidates to mistakenly think MED always influences path selection or that the value itself is the issue.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MED is only compared when paths are from the same neighboring AS.
BGP's MED (Multi-Exit Discriminator) attribute is only compared between paths that originate from the same neighboring AS. In this scenario, even though R2 receives a route with MED 50 from R1, the path is not preferred because the competing path likely comes from a different neighboring AS, making the MED comparison invalid. MED is a non-transitive attribute that influences inbound traffic only when comparing multiple exit points from the same AS.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The MED value is too low to influence path selection.
Why it's wrong here
MED is only compared between paths from the same neighbouring autonomous system; R2 received the route from a different AS, so the value is discarded before best-path selection. It tempts because 50 is numerically low, but MED has no absolute scale — only relative comparison within one AS applies.
- ✗
The route-map should be applied outbound, not inbound.
Why it's wrong here
MED is compared only between routes from the same neighbouring AS, so setting it inbound on R1 does not influence R2's selection here. Outbound application would be correct when the local router advertises routes to an external peer and must influence that peer's entry path.
- ✓
MED is only compared when paths are from the same neighboring AS.
Why this is correct
BGP compares MED only between paths originating from the same neighbouring autonomous system. R2 received the MED 50 route from a different AS than the competing path, so the attribute is ignored during best-path selection, leaving that route unpreferred despite the lower metric.
- ✗
The neighbor has a higher local preference overriding MED.
Why it's wrong here
Local preference is exchanged only within an autonomous system via IBGP; R2 receives this route from an external ISP peer, so no local preference value from R1 overrides the MED. Local preference would decide if both paths arrived via IBGP inside one AS.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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