In L3 spine and leaf design

Question

Options

• ASpanning Tree is always needed since Spine and Leaf is only there to improve performance
• BYou need to use TRILL to prevent loops
• CYou don't need to use spanning tree for loop prevention
• DSince all Spine switches are connected to each other, EVPN is needed in this design

• A
  3% (1)
• B
  3% (1)
• C
  90% (35)
• D
  5% (2)

Explanation

In an L3 spine-leaf fabric, every link between a leaf and a spine is a routed (Layer 3) link, not a bridged Layer 2 connection. Because IP routing has built-in loop prevention (TTL decrement, routing protocol metrics), STP - a Layer 2 mechanism - is simply irrelevant; there are no bridging loops to prevent.

Why the distractors are wrong:

• A is wrong on both counts: spine-leaf isn't only about performance, and STP is not "always needed" - L3 routing eliminates that requirement entirely.
• B is wrong because TRILL is a Layer 2 alternative to STP used in L2 fabrics; it has no role in an L3 design where routing already handles loop prevention.
• D contains a factual error: in standard spine-leaf, spine switches are not connected to each other (only leaves connect to all spines). EVPN is also an optional overlay for services like VXLAN/L2 extension, not a baseline requirement.

Memory tip: Let the "L3" prefix do the work - if it's Layer 3 (routed) everywhere, routing protocols handle loops, so STP is out of a job. "L3 = Route it, don't bridge it, don't STP it."

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