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JNCIE-DC study materials
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JNCIE-DC: Layer 2 and Layer 3 Underlay and Overlay Networking

JNCIE-DC · Work through your exam, one topic at a time.

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Layer 2 and Layer 3 Underlay and Overlay Networking

Domain 3 covers the two layers beneath every tenant: the IP underlay that makes VTEP loopbacks reachable with ECMP, and the BGP EVPN overlay that distributes MAC/IP and prefix reachability.

Underlay rules

  • eBGP leaf-to-spine, one session per link, family inet unicast, loopback /32 advertised through an export policy.
  • multipath multiple-as so equal-cost paths install; confirm with show route <vtep> showing two next hops.
  • BFD on every fabric link so a dead link is removed in milliseconds rather than at BGP hold time.
  • Jumbo MTU 9000 on fabric links, at least 1550 on host-facing VTEP ports.

Overlay rules

iBGP with the spines as route reflectors is the common design: every leaf must receive every other leaf's Type 2 and Type 3 routes, so the RR group needs clients and a cluster ID. family evpn signaling must be negotiated. MAC learning is control-plane; the data plane is not how MACs are discovered. The L3 overlay uses Type 5 routes in a VRF with its own L3 VNI and vrf-target.

Worked example

Leaf1's loopback is 10.0.0.1/32, Leaf2's is 10.0.0.2/32. Leaf1 advertises its /32 to spine1 and spine2; each spine selects it best and re-advertises; Leaf2 receives two paths and, with multipath multiple-as, installs both as the VXLAN outer destination. Remove the export policy on one spine and the path count drops to one - the fastest proof that the underlay is symmetric.

Traps

Full-mesh iBGP because the reflector was skipped; assuming show bgp summary up means forwarding works (check show route forwarding-table for the VTEP prefix); hash asymmetry pinning one spine link near saturation; MTU left at 1500 on a transit link so large frames die silently in the middle of the fabric.