“Fabric” is a loosely used term, which today creates more confusion instead of offering direction.
What exactly is a Fabric ? What is a Switch Fabric?
Greg Ferro did a post here explaining how Ethernet helped the layer 2 switch fabric evolve. Sadly the use of switch fabric did not stop there. And this is the part where the confusion trickles in.
The term fabric has been butchered (mostly by marketing people) to incorporate just about any function these days. The term ‘switch fabric’ today (in the networking industry) is broadly used to describe among others the following:
- The structure of an ASIC, e.g., the cross bar silicon fabric.
- The hardware forwarding architecture used within layer2 bridges or switches.
- The hardware forwarding architecture used with routers, e.g., the Cisco CRS and its 3-stage Benes switch fabric.
- Storage topologies like the fabric-A and fabric-B SAN architecture.
- Holistic Ethernet technologies like TRILL, Fabric-Path, Short-Path Bridging, Q-Fabric, etc.
- A port extender device that is marketed as a fabric extender (a.k.a. FEX) namely the Cisco Nexus 2000 series.
In short, a switch fabric is basically the interconnection of points with the purpose to transport data from one point to another. These points, as evolved with time, could represent anything from an ASIC, to a port, to a device, to an entire architecture.
Cisco added a whole new dimension to this by marketing a Port Extender device as a Fabric Extender and doing so with different FEX architectures namely VM-FEX and Adapter FEX…. More on that in the next post. :)
In this post I would like to cover the base of what is needed to know about the Cisco Fabric Extender that ships today as the Nexus 2000 series hardware.
The Modular Switch
The concept is easy to understand referencing existing knowledge. Everybody is familiar with the distributed switch architecture commonly called a modular switch:
Consider the typical components:
- Supervisor module/s are responsible for the control and management plane functions.
- Linecards or I/O modules, offers physical port termination taking care of the forwarding plane.
- Connections between the supervisors and linecards to transport frames e.g., fabric cards, or backplane
- Encapsulating mechanism to identify frames that travel between the different components.
- Control protocol used to manage the linecards e.g., MTS on the catalyst 6500.
Most linecards nowadays have dedicated ASICs to make local hardware forwarding decisions, e.g., Catalyst 6500 DFCs (Distributed Forwarding Cards). Cisco took the concept of removing the linecards from the modular switch and boxing them with standalone enclosures. These linecards could then be installed in different locations connected back to the supervisors modules using standard Ethernet. These remote linecards are called Fabric Extenders (a.k.a. FEXs). Three really big benefits are gained by doing this.
- The reduction of the number of management devices in a given network segment since these remote linecards are still managed by the supervisor modules.
- The STP footprint is reduced since STP is unaware of the co-location in different cabinets.
- Another benefit is the cabling reduction to a distribution switches. I’ll cover this in a later post. Really awesome for migrations.
Lets take a deeper look at how this is done.
Continue reading “What is a Fabric Extender”