Question 1,102 of 2,152
Device Access ControlhardMultiple ChoiceObjective-mapped

BGP Path Selection: Always-Compare-MED Overriding AS Path Length

This 300-410 practice question tests your understanding of device access control. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. A key principle to apply: bGP best path selection. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

A BGP-speaking router R1 is experiencing unexpected path selection for prefix 10.0.0.0/8. R1 receives two BGP updates: one from neighbor 192.168.1.2 with local preference 150, AS path 65001 65002, and MED 50; another from neighbor 192.168.2.2 with local preference 100, AS path 65001, and MED 100. R1's BGP configuration includes: bgp always-compare-med. The show ip bgp 10.0.0.0/8 output shows the path via 192.168.1.2 as best, but the network team expects the path via 192.168.2.2 to be best due to shorter AS path. What is the root cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

  • Clue: "always"

    Why it matters: Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.

Quick Answer

The answer is that the `bgp always-compare-med` command causes the router to prefer the path with MED 50 over the shorter AS path, making the route via 192.168.1.2 the best. This command forces BGP to compare Multi-Exit Discriminator (MED) values even when the paths originate from different neighboring autonomous systems, which normally would skip MED comparison. In this scenario, without the command, the shorter AS path of 65001 would win, but with `bgp always-compare-med` enabled, the lower MED of 50 overrides the AS path length during path selection. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap: candidates often forget that `bgp always-compare-med` changes the BGP best-path algorithm order, allowing MED to be compared before AS path length when the paths come from different ASes. A helpful memory tip is “MED overrides AS length when always-compare is in effect,” reminding you that this command elevates MED’s priority in the decision process.

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

The local preference on the path via 192.168.1.2 is higher (150 vs 100), overriding AS path length.

BGP best path selection compares local preference before AS path length. The route from 192.168.1.2 has local preference 150, which is higher than the local preference 100 from 192.168.2.2. Therefore, the route via 192.168.1.2 is preferred regardless of AS path length or MED. The `bgp always-compare-med` command is irrelevant because local preference is evaluated before MED.

Key principle: BGP best path selection

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 bgp always-compare-med command causes MED comparison across different AS paths, making the path with lower MED (50) preferred over the shorter AS path.

    Why it's wrong here

    This is incorrect. While `bgp always-compare-med` enables MED comparison across different ASes, local preference is compared before both AS path length and MED. Since local preference differs (150 vs 100), the path with higher local preference is chosen first, making MED irrelevant.

  • The local preference on the path via 192.168.1.2 is higher (150 vs 100), overriding AS path length.

    Why this is correct

    This is correct. Local preference is the first tiebreaker after weight and locally originated routes. The path via 192.168.1.2 has a higher local preference (150), so it is selected as best, explaining why the shorter AS path is not chosen.

    Clue confirmation

    The clue word "always" in the question point toward this answer.

    Related concept

    BGP best path selection

  • The MED value of 50 on the first path is lower, but without always-compare-med, the second path would be best due to shorter AS path. However, the command is not present, so the behavior is normal.

    Why it's wrong here

    This is incorrect. The `bgp always-compare-med` command is present, but its effect is not the root cause. Even without the command, the higher local preference would still determine the best path.

  • The AS path length is not considered because the paths have different neighbor AS; BGP prefers the path with the lower neighbor AS.

    Why it's wrong here

    This is incorrect. BGP does not use neighbor AS as a tiebreaker in the standard best path selection process. AS path length is compared after local preference, not neighbor AS.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the BGP best path selection order, especially the interaction between local preference, AS path length, and MED with 'bgp always-compare-med'. Candidates may mistakenly think AS path length is compared before local preference or forget that MED is only compared with this command across different ASes.

Trap categories for this question

  • Command / output trap

    This is incorrect. The `bgp always-compare-med` command is present, but its effect is not the root cause. Even without the command, the higher local preference would still determine the best path.

Detailed technical explanation

How to think about this question

Under the hood, BGP best-path selection follows a strict order of tie-breakers: highest weight, highest local preference, locally originated, shortest AS path, lowest origin type, lowest MED (if same neighboring AS or `bgp always-compare-med` is configured), and so on. The `bgp always-compare-med` command modifies step 6 of the decision process, allowing MED comparison between paths from different ASes, which can override a shorter AS path. In real-world scenarios, this command is used cautiously because it can cause suboptimal routing if MED values are not consistently set across autonomous systems.

KKey Concepts to Remember

  • BGP best path selection
  • bgp always-compare-med

TExam Day Tips

  • Watch for words such as best, first, most likely and least administrative effort.
  • Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

BGP best path selection

Real-world example

How this comes up in practice

A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.

What to study next

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FAQ

Questions learners often ask

What does this 300-410 question test?

Device Access Control — This question tests Device Access Control — BGP best path selection.

What is the correct answer to this question?

The correct answer is: The local preference on the path via 192.168.1.2 is higher (150 vs 100), overriding AS path length. — BGP best path selection compares local preference before AS path length. The route from 192.168.1.2 has local preference 150, which is higher than the local preference 100 from 192.168.2.2. Therefore, the route via 192.168.1.2 is preferred regardless of AS path length or MED. The `bgp always-compare-med` command is irrelevant because local preference is evaluated before MED.

What should I do if I get this 300-410 question wrong?

Review bGP best path selection, then practise related 300-410 questions on the same topic to reinforce the concept.

Are there clue words in this question I should notice?

Yes — watch for: "always". Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.

What is the key concept behind this question?

BGP best path selection

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Last reviewed: Jun 24, 2026

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