What is a design consideration for using IBGP for the underlay in a Clos data center fabric?

Question

Options

• AA full mesh of IBGP neighbors will be required.
• BBGP ADD-PATH must be enabled for multipath to work on leafs.
• CAn IGP will always be required for learning loopback addresses.
• DA 5-stage Clos topology will not work with IBGP underlay.

• A
  13% (2)
• B
  73% (11)
• C
  7% (1)
• D
  7% (1)

Explanation

Option B is correct because IBGP's split-horizon rule causes routers to advertise only the single best path to IBGP peers by default. In a Clos fabric where leafs have multiple equal-cost paths through different spines, without BGP ADD-PATH (RFC 7911), a leaf would only receive one path per prefix and lose ECMP capability - ADD-PATH allows multiple paths for the same prefix to be advertised, restoring multipath load balancing.

A is wrong because spines in a Clos fabric typically act as Route Reflectors for the leaves, eliminating the need for a full IBGP mesh. C is wrong because IBGP sessions can be established over directly connected physical interfaces rather than loopbacks, making an IGP unnecessary - this is a common Clos design pattern. D is wrong because there is no architectural limitation preventing IBGP from functioning in a 5-stage Clos topology; Route Reflectors can be deployed at multiple tiers.

Memory tip: Think "IBGP only shares its BEST - ADD-PATH shares the REST." Whenever you see IBGP + Clos + multipath in the same sentence, ADD-PATH is the answer.

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