Ethernet Fabric (VDX, CNA)

Retired-Super Contributor
Posts: 260
Registered: ‎05-12-2010

VCS Ethernet fabrics and STP

Spanning Tree is one of the classic Ethernet issues that Brocade VCS technology tackled.   An Ethernet fabric does not need STP to prevent loops.  Instead, it uses link state routing, of which there are several implementations to choose from.  Link state routing protocols are traditionally used at layer 3.  An Ethernet fabric implements link state routing at layer 2 to create a map of neighbor switches and to learn the path cost between switches in an arbitrary topology.  Frames are always forwarded along the shortest path, and if multiple paths exist between switches with the same path cost, their aggregate bandwidth is available for inter-switch traffic.  This is a huge win over STP which always prunes inter-switch links to ensure only one active path exists between switches.

Okay, but for a technology revolution like Ethernet fabrics to be successful, it has to peacefully co-exist with legacy technology such as STP.  How does a Broade VDX 6720 switch support co-existence?  Well in two ways depending on how you configure it.

It may have escaped your notice, but the VDX 6720 can be used in "standalone" mode which means it does not participate in an Ethernet fabric.  In that case, it supports various flavors of STP as shown here.  There are several customer use cases where a very low latency (600 ns port-to-port for the 6720-24 model) 1GbE/10GbE top of rack switch with very low power consumption is an ideal solution.

Now, when a VDX 6720 is running in VCS mode (which is the default by the way), then connecting two or more together automatically creates an Ethernet fabric and there is no STP used, instead link state routing is used to remove forwarding loops.

So, what happens if a VDX has edge ports connected to some other Ethernet switch not running in VCS mode?  Wouldn't that switch rely on STP?  Certainly it could.  Now what happens?

In this case, the Ethernet fabric of VDX switches behaves as a transparent LAN service (TLS) to the STP BPDU (Bridge Protocol Data Units) forwarding them as if they are data frames.  Said differently, the edge port of a VDX switch does nothing with any frame with STP BPDU contents except forward it across the fabric to its destination.  Clearly, the destination is some other classic Ethernet switch running STP, so there is no need for a VDX switch in VCS Ethernet fabric mode to process it.  From the perspective of any classic Ethernet switches exchanging STP BPDU frames, the VCS Ethernet fabric is invisible.

To wrap up, if a VDX is configured to run in standalone mode, it uses STP to form a loop free network with its neighbors.  If a VDX is in the default mode (VCS Ethernet fabric mode), then it forwards BPDU frames and is invisible to any classic Ethernet switch using STP.

I hope that makes sense.   Let me know if you have any question on this topic.

Posts: 52
Registered: ‎02-18-2010

Re: VCS Ethernet fabrics and STP

Can you explain a little more about the link-state routing protocols used on the VDX to create a loop free fabric?

Thank you,


Posts: 61
Registered: ‎12-08-2009

Re: VCS Ethernet fabrics and STP

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