The core is the problem

A traditional switched network keeps the core loop-free with spanning tree, which means blocking links to break loops, and it layers protocol on protocol to deliver services: 802.1Q trunks for VLANs, maybe MPLS or VXLAN with BGP for virtualization, plus something to move multicast. Each layer is another thing to design, provision at every hop, and troubleshoot. VOSS - Extreme's Fabric Connect - was built to collapse that.

What Fabric Connect is

Fabric Connect is Extreme's implementation of Shortest Path Bridging MAC, or SPBM, standardized as IEEE 802.1aq (with IP extensions in IETF RFC 6329). It has a long heritage from ex-Nortel and Avaya, who were early and thorough SPBM implementers. The idea is to reduce the core to a single Ethernet-based link-state protocol that provides all virtualization services in one integrated model, instead of a stack of overlays.

Two pieces make that work:

  • a data plane based on MAC-in-MAC: customer frames are encapsulated in a backbone MAC header and switched across the core on backbone MAC addresses (B-MACs), so the core never inspects the customer frame; and
  • a control plane based on IS-IS: nodes form adjacencies, discover the topology, compute shortest paths to every other node, and advertise service membership - all in one protocol.

No spanning tree in the core

SPBM does not run spanning tree across the fabric. Transport uses two Backbone VLANs (B-VLANs, commonly 4051 and 4052). A B-VLAN is not a normal VLAN: it never floods unknown, broadcast, or multicast traffic. It forwards strictly on the backbone-MAC tables that IS-IS builds, following the shortest path IS-IS computed. Because forwarding is computed rather than learned by flooding, every link can carry traffic and convergence is fast.

Provision the edge, leave the core alone

The switches at the boundary of the fabric are Backbone Edge Bridges (BEBs); the transit switches in the middle are Backbone Core Bridges (BCBs). All service configuration happens on the BEBs, where the application meets the network - you attach a VLAN or a routing instance to a service there, and IS-IS floods that membership so every relevant BEB participates. The BCBs in between are provisioned once for the fabric itself and then left alone; they carry any service without per-service configuration. This is the SPBM promise often summarized as "build the network once and do not touch it again."

Where to go next

The unit of service on the edge is the I-SID, covered in the companion article on I-SIDs and VSNs. The control plane - IS-IS, node nicknames, and B-MACs - is covered in the article on the VOSS control plane. The fabric-identifier tool decodes the I-SIDs, nicknames, and B-MACs you will meet along the way.