Data Center Networking

I lost an afternoon to a “routing loop” that turned out not to be a loop at all. It was an EVPN all-active multihoming feature quietly not working in a virtual lab — and the packets ping-ponging between two switches were a symptom, not the disease. This post is two things: a refresher on how Arista ESI (Ethernet Segment) multihoming actually forwards traffic, and a write-up of the specific trap you can fall into when you build EVPN designs on vEOS/containerlab instead of real hardware. ...

In an EVPN/VXLAN fabric, the MAC address table is part of the control plane: when a MAC is learned or removed, the switch generates a BGP EVPN update so the rest of the fabric stays in sync. That tight coupling is exactly what makes EVPN work — but it also means that ordinary MAC aging on a quiet host can turn into a stream of BGP churn. This post is about a problem we chased down to the interaction between ARP timeout and MAC address-table timeout, and why the common assumption about “any packet refreshes the timer” is wrong. ...