BGP Local_Pref Attribute Scope and Priority Rules

H12-821 By islever Editorial Last updated

BGP Path Attributes and RR

Short answer

AB The Local_Pref attribute maintains its value within the AS unless modified by policy, and a higher value indicates a more preferred route.

Question

Which of the following statements are true about the Local_Pref attribute in BGP? (Choose all that apply.)
  1. The Local_Pref value remains unchanged during transmission unless it is affected by a routing policy. Correct Answer
  2. The greater the value of the Local_Pref attribute, the higher the priority. Correct Answer
  3. The Local_Pref attribute can be transmitted to all BGP peers.
  4. The default Local Pref value is 0.

Key Takeaways

The main trap is confusing Local_Pref with attributes that traverse AS boundaries (like AS_Path) or misremembering the default value. Remember: Local_Pref is for inbound traffic control within a single AS.

This question tests understanding of the BGP Local_Pref attribute's intra-AS scope and its role in route preference. The correct statements confirm that Local_Pref is higher-is-better and stays unchanged unless policy modifies it.

Common mistake: Candidates often select C, mistakenly believing Local_Pref is advertised to eBGP peers as well. However, Local_Pref is strictly an iBGP-only attribute used to coordinate exit points within the local AS.

Expert Analysis

The Local_Pref attribute is a critical component of BGP path selection within an Autonomous System (AS). It is classified as a Well-Known Discretionary attribute, meaning all BGP speakers must recognize it, but its advertisement behavior is discretionary. The defining characteristic of Local_Pref is its scope: it is strictly an intra-AS attribute. This means it is exchanged only between iBGP peers and is never advertised to eBGP peers. Consequently, statement C is incorrect because Local_Pref does not propagate to 'all' BGP peers; specifically, it stops at the AS boundary.

Regarding value semantics, BGP uses Local_Pref to influence inbound traffic engineering. The protocol dictates that a higher Local_Pref value indicates a more preferred path. Therefore, statement B is correct, as the greater the value, the higher the priority for route selection among competing paths within the same AS.

Statement A is also correct. While Local_Pref can be modified by routing policies (such as route-maps or prefix-lists) on ingress routers, once set, the attribute value remains consistent across the iBGP mesh unless explicitly changed by configuration. It preserves its value during transmission within the AS.

Statement D is factually wrong. The default Local_Pref value in standard BGP implementations (including Huawei, Cisco, etc.) is 100, not 0. A value of 0 would typically indicate a very low preference or invalid state, which contradicts standard operational defaults.

In summary, the key distinction here is scope (iBGP only vs. all peers) and value logic (higher is better). Misunderstanding these leads candidates to incorrectly select C (thinking it propagates globally like AS_Path) or D (confusing it with other metrics).

Exam Strategy

When analyzing BGP attributes, always first determine if the attribute is transitive and whether it crosses AS boundaries. For Local_Pref, immediately rule out any option suggesting eBGP propagation. Second, recall the numeric direction: for Local_Pref and MED (in some contexts), higher is often more preferred or significant, whereas for Administrative Distance or Cost, lower is usually better. Memorize the default values (Local_Pref=100, MED=0) to quickly eliminate distractors.

Frequently Asked Questions

Can Local_Pref be sent to eBGP peers?

No. Local_Pref is a Well-Known Discretionary attribute that is only exchanged between iBGP peers. It is never advertised to eBGP peers to prevent influencing external AS path selection.

What happens if Local_Pref is not set?

If no Local_Pref is explicitly configured, BGP assigns a default value of 100 to the route. This ensures a baseline preference level for all internal routes.

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Community Votes

A
50%
B
50%

50% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 1 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 1

  1. HCIP Resolutor 👍 1
    C. The Local_Pref attribute can be transmitted to all BGP peers: Es una afirmación FALSA. El atributo Local_Pref es de tipo Well-Known Discretionary, pero solo tiene alcance dentro del propio Sistema Autónomo (Intra-AS). Se transmite únicamente a pares IBGP y NUNCA a pares EBGP.

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