In a multi-AS underlay design

Question

Options

• AAll spine switches are in the same AS while each leaf (VSX pair if used) are in their own AS
• BEvery spine and every leaf is in their own AS
• CAll spine and leaf switches are in one AS and the border leaf is in separate AS
• DEvery spine is in their own AS while all leaf switches in one AS

• A
  87% (34)
• B
  3% (1)
• C
  8% (3)
• D
  3% (1)

Explanation

In a multi-AS underlay design, all spine switches share a single AS number while each leaf switch (or VSX pair, if used) is assigned its own unique AS. This structure creates eBGP sessions between every leaf and the spines, enabling loop-free routing and simplified path selection across the Clos fabric - the defining characteristic of a multi-AS underlay.

Why the distractors are wrong:

• B is incorrect because giving every spine its own AS is unnecessary and atypical; spines are grouped into one AS to keep the design scalable.
• C describes a single-AS design for the core fabric, which is the opposite of multi-AS; it conflates the role of the border leaf with the underlay AS structure.
• D reverses the actual design - it's the leaves (not spines) that each get their own AS, since leaves are the attachment points for individual racks/servers needing distinct routing domains.

Memory tip: Think "Spines Share, Leaves Leave Alone." The spine layer is a single shared AS (like a shared backbone), while each leaf breaks off into its own AS - mirroring how leaves branch individually from a shared trunk.

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