Evpn

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. ...

ESI Multi-Homing and ARP Suppression Issue: Duplicate Packets & Flooding When ESI is not functioning correctly, you may observe duplicate packets destined for hosts behind the ESI. Remote switches might flood traffic toward the ESI switches instead of handling it efficiently. TCAM Carving for ARP Suppression To use the suppress-arp feature effectively and prevent unnecessary flooding, you must allocate sufficient TCAM resources on the switch hardware. 1 2 3 # Configure TCAM carving for ARP suppression hardware access-list tcam region racl 512 hardware access-list tcam region arp-ether 256 double-wide Warning: It was observed that enabling suppress-arp caused reachability and ARP resolution to break between hosts when attempting to use RFC 5549 (BGP Unnumbered / IPv6 underlay) between Spines and Leafs for IPv4 and EVPN. Note that an IPv6 underlay is generally not supported for Cisco Nexus 9300/9500v platforms in this context. ...

Why anycast inside the fabric is harder than it looks An anycast service is advertised from multiple hosts into the data center fabric — the same prefix, the same service IP, originated from several places at once. The promise is appealing: multiple next hops for one service prefix gives you load balancing, fast failover, and horizontal scalability without any state in the network. The catch is that for every leaf to actually use all of those origins, the fabric has to carry and install multiple equal-cost paths for a single prefix end to end. That is not BGP’s default behavior, and the moment you combine it with two things modern fabrics love — an IPv6 unnumbered underlay and IPv4-in-IPv6 next hops (RFC 5549) — one of the most common EVPN disambiguation tricks stops working. This post walks through why, and what the realistic options are. ...

Companion posts. This is the hands-on lab — real NX-OSv configs, the tests I ran, and where they broke. For the high-level summary see Anycast Inside the Data Center; for the deeper protocol write-up (overlay index, recursive resolution, why pure-IP fabrics lose per-host ECMP) see Anycast in EVPN/VXLAN Fabrics: The IPv6 Unnumbered and RFC 5549 Problem. Why Anycast is used for horizontal scaling and for better failure response. With modern orchestration and virtualization, a new instance of a service can be spawned on demand — by an engineer or by the system itself. ...

Part of an anycast series. This is the high-level summary. For the hands-on NX-OS lab (configs, the RFC 5549 failure, the working EVPN solution) see Anycast Inside an EVPN/VXLAN Fabric with BGP Unnumbered (RFC 5549), and for the deeper protocol treatment see Anycast in EVPN/VXLAN Fabrics: The IPv6 Unnumbered and RFC 5549 Problem. Why Anycast services are advertised from multiple hosts into the data center. The same service prefix is originated from several places at once, which gives you multiple next hops for that service — and with it, load balancing, better failover, and scalability. ...

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. ...