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Zorluk: OrtaRouting Concepts and Dynamic Routing Protocols

An edge router receives multiple exterior gateway protocol routes pointing to the same destination network prefix from different autonomous systems. Place the Border Gateway Protocol (BGP) path selection attributes in the correct order of evaluation from first to last.

  1. 1Highest Local Preference
  2. 2Shortest AS-Path length
  3. 3Lowest Origin type (IGP preferred over EGP, which is preferred over Incomplete)
  4. 4Lowest Multi-Exit Discriminator (MED)

Cevap

The correct order of evaluation for BGP path selection attributes from first to last is: Highest Local Preference, Shortest AS-Path length, Lowest Origin type, and Lowest Multi-Exit Discriminator (MED).
BGP path selection follows a deterministic hierarchy to pick the best path when multiple routes exist for a prefix. The evaluation begins with local attributes (Highest Local Preference), proceeds to path metric attributes (Shortest AS-Path length), checks route source reliability (Lowest Origin type), and finally compares entry points for neighboring systems (Lowest MED).

Adım Adım Çözüm

1
Identify the primary outbound policy attribute.
Highest Local Preference is selected as the top criteria evaluated within an Autonomous System.
Local preference takes precedence over path length and origin characteristics to allow local administrators to override default routing decisions.
2
Evaluate path distance metric when local preferences are equal.
Shortest AS-Path length is evaluated second.
BGP prefers routes that cross fewer autonomous systems to minimize inter-domain transit latency.
3
Compare route origin source mechanism.
Lowest Origin type is evaluated third.
Routes created natively via internal routing protocols (IGP) are trusted more than external or unknown redistributed routes.
4
Determine path preference into neighbor autonomous systems.
Lowest Multi-Exit Discriminator (MED) is evaluated fourth.
MED acts as a tie-breaker when multiple entry points exist into the same neighboring autonomous system.

Anahtar Kavram

BGP Path Selection Algorithm Hierarchy
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Konular

Routing Concepts and Dynamic Routing Protocols

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