As a result of my recent post on Stackable vs Chassis Based Switches I was looking Juniper Virtual Chassis Technology and found some really interesting ideas on the architecture.

Intro to Juniper Virtual Chassis Technology

From the VIRTUAL CHASSIS TECHNOLOGY BEST PRACTICES here is a brief description

Juniper Networks® Virtual Chassis technology is a feature of the Juniper Networks EX4200 line of Ethernet switches allowing the interconnection and operation of switches as a unified, single, high bandwidth device. Up to 10 EX4200 switches may be interconnected via dedicated Virtual Chassis ports on each device, or through optional uplink module ports that are configured as Virtual Chassis ports, with a combined backplane bandwidth of up to 128 Gbps.

OK so you get the idea. A stack connection is always a fat bandwidth connection between each switch that attempts to extend the switching backplane to a much larger physical space than just a few centimetres between chips, or a 75 centimetre haul down a backplane. However, the provision of high bandwidth over a cable or a backplane is limited by a number of physical problems such as clocking, signal waveform and importantly, distance. You can’t beat the speed of light, and it’s hard to get a square waveform at high frequencies.

But the Juniper stack connection must have a lot of parallel bandwidth in there, since they are getting 64Gbps in their connection (note that is twice as fast as a Cisco C3750 stack connection). But interestingly, the Juniper Stack connector cable can be up to 5 metres in length. This opens up a number of interesting design possibilities.

For example, the Cisco C3750 has a long stack cable that can be 75cm long, and is used when you stack gets bigger than four units. Without that ‘extra special’ cable, you can’t get the stack loop installed. This means that a Cisco C3700 stack must always be adjacent and you don’t have a lot of choices for cable management.

Design Options

So lets look at the design options proposed by Juniper in the best practices document.

Daisy chained Ring

This is pretty much the standard layout for a stack. Short distance between one leg and a long return cable to complete the redundant path.

Braided Ring

This is different. I’m guessing that the ring arbitration system doesn’t need the stack to be in any particular order. Combine that with an electrical signalling that allows high speed (in this case 64Gbps on that cable) for up to 5 metres and you can actually have those switches with an end to end span of 22.5 metres.

Top of Rack, Daisy Chain

Lets turn that Braided Ring into something practical. What about a top of rack design for low density server zone.

Figure 15 shows an example of extending the data center, top of rack deployment across several racks. This figure illustrates a braided-ring cabling configuration, which allows spacing of up to 5 meters between alternate racks.

Caption Text.(Click for a full size image)

In this practical scenario, this means a 48-port switch in the top of each rack with up to ten racks in a row. Multiple uplinks can can be allocated according to the bandwidths requirements as needed. For certain low ethernet densities this a very interesting solution. I can imagine using a design that has this setup twice. That is, I have two switches at the top of every rack, but connected as show here to provide dual switch redundancy. And I’m thinking that this would be a cost effective solution.

Not a fan of Stackable Switching

I recently wrote about Chassis vs Stackable switches and that I don’t think Stackable switches are very good. But the design convenience of this option with ease of use and low cost, might outweigh my concerns about reliability. And since this type of design is only going to appeal to low end, low cost customers then it might well be the correct choice.

The Etherealmind View

It’s always interesting to look at other vendors equipment and learn about the different techniques to solve problems. This design shows that Cisco doesn’t have all the answers and other vendors have solutions that can be very different, and better. Note to self: must look at other vendors more often [grits teeth], even if that is a lot of work.