BGP

A GPU cluster that serves models has three kinds of user and one hard question: which layer owns which decision? Two complete builds walked layer by layer, down to the configuration, with the one choice that changes everything: rails as plain VLANs, or a routed rail Clos.

Three ways to make two Nexus 9000 leaves one LACP partner: vPC, vPC Fabric Peering, EVPN ESI multihoming. All forward on both cables. Which one holds up when a leaf loses the fabric, the host leg dies, or the peer-link goes? Four pairs, one fabric, thirteen drills, three passes.

EVPN multihoming vs MLAG: what actually breaks Both make two leaf switches look like one LACP partner to a dual-homed server. Both forward on both links. So which one holds up when something fails? Two labs on Cumulus Linux, identical wiring, four failure scenarios, every probe accounted for.

Type-2 gets all the attention, but a datacenter fabric runs on five core EVPN route types and quietly depends on every one. Here is what each actually carries, decoded from a live multihomed fabric, including the two types MLAG labs never get to show you.

Anycast usually lets the network decide which datacenter answers. I moved that decision onto two GoBGP hosts that watch both datacenters and steer a single global VIP between them. Here's the design, the traffic flow, and three failover drills that came through with zero failed requests.

Two elephant flows, six trials, six collisions: one uplink carried everything while its twin sat at zero, and every dashboard stayed green. The measured polarization, the four-witness diagnostic chase, and the birthday math production fabrics live with.

The finale: one request traced live through all eight hops the series built, the five traps ranked by hours lost, and what I would do differently next time. Plus the honest single point of failure the design still carries.

Wiring Active/Active firewalls into the fabric: four eBGP sessions, thirteen prefixes up and one default down, device-bound NAT, two DNAT pinholes for all inbound traffic, and the vPC SVI timer lesson.

VXLAN tunnels between loopbacks, EVPN as the directory, anycast gateways everywhere, and the route-target auto trap that silently empties eBGP fabrics. With the complete overlay configs for all twelve switches.