Data Center Architecture and Design
Domain 1 tests whether you can choose the topology, addressing plan, and overlay model that every later configuration task assumes.
Topology rules
A spine-leaf Clos fabric: every leaf connects to every spine, no leaf-to-leaf links, so any two hosts are a fixed, known number of hops apart and ECMP behaves deterministically. The underlay is normally eBGP with a unique ASN per leaf and a shared spine ASN; multipath multiple-as turns two spine paths into one ECMP set. STP is not used, because loops are prevented at Layer 3.
MTU budget
VXLAN adds 50 bytes of outer encapsulation (outer IPv4 20, UDP 8, VXLAN 8, plus outer Ethernet framing). Fabric links should carry 9000; a VTEP port facing 1500-byte hosts needs at least 1550, commonly configured as 1600.
Routing models
Centralized routing keeps a border-leaf pair as the gateway. Distributed routing (CRB) puts an anycast IRB on every leaf with virtual-gateway-address and virtual-gateway-vmac, so hosts always have a local first hop and the VM can move anywhere. Symmetric IRB carries a per-VRF L3 VNI; asymmetric IRB does not.
BUM handling
Ingress replication is the simplest option, static or driven by EVPN Type 3 with multicast-mode ingress-replication. A multicast underlay offloads replication to the network but adds PIM and group management.
Worked example
Two spines in AS 65100, four leaves in 65001-65004, /31 fabric links, /32 loopbacks. Leaf1 advertises its loopback to both spines; each spine selects it best and passes it on; Leaf1 installs two paths thanks to multipath multiple-as. VTEP reachability is now guaranteed for every VXLAN tunnel, before a single VNI is configured.
Traps
Leaving fabric interfaces at 1500, assuming VXLAN requires a multicast underlay, adding leaf-to-leaf links for redundancy, and forgetting that anycast gateway requires an identical virtual-gateway MAC on every leaf.