Understanding Switch Fabrics Part 2

In the last post of this series we introduced how Brocades Ethernet fabrics function from a high level and we discussed the need for it. I promised that in this post we would begin to discuss how the Brocade VDX switch works in it's two operational modes, Classic Mode and VCS mode. We will not be spending much time on the Classic Mode, mainly because most already understand this mode. In Classic mode you operate as a standard ethernet switch. This means things like Layer 2 forwarding, MAC learning, STP and so on are in the mix. Since we want to get away from that mix lets focus on VCS mode. VCS mode has many of the capabilities of a Classic Switch but there are some additions. These additions include Transpared LAN Service, vLAGs, Distributed Config Management, and End-to-End FCoE. We will discuss these features as we make our way through. For now, recall that in the Part 1 post we talked about how a cluster is formed and we had edge ports that connect to non-VCS mode switches and internally we have Fabric ports negotiated between switches that operate in VCS mode. All of the fabric ports were transparent to those on the outside of the cluster. The determining factor on whether a switch operates in VCS mode is a setting that is user controlled. If a switch comes up with the VCS mode set to OFF it assumes the Classic Switch mode and all the ports would be defaulted to a shutdown state.

If the a switch is configured with VCS mode enabled when it powers up then all the ports transition from shutdown state and start operating as edge ports. Next, the Brocade Proprietary protocol called FLDP, or Fabric Link Discovery Protocol, discovers other switches that are operating in VCS mode and if the neighbor is a Brocade VDX switch. An illustration of this is seen in the image below.


If the neighbor is not a VDX switch the port will become an edge port. If the neighbor is a VDX Switch we then want to know if its operating in VCS mode. If it is operating in VCS mode the VCS ID must match. Each VCS fabric is identified by a VCS ID which defaults to 0. Assuming that the neighboring VDX switch has the same VCS ID the port transitions to a fabric port and an ISL is established.

Brief Mention of TRILL

Trill is used within the Fabric (with some proprietary Brocade functions) and every switch assumes itself to be a principle RBridge. The RBridge ID's must be unique and the principle Rbridge determines if this is happening. If there is a conflict then the principle switch keeps the joining switch with a duplicate RBridge ID segmented. At this point the RBridge ID on one of the conflicting switches needs to be changed and if the ID is changed the switch needs to be rebooted for it to take effect.

Control Plane VLANs

There are two control plane VLANS used for control traffic. There is an Edge Control VLAN (4095) and a Fabric Control VLAN (4093). These are both created upon boot up of the switch. The edge control VLAN is what tunnels control frames between two edge ports across the fabric. Recall in the last post we mentioned that BPDU's were transparently passed across the fabric. The Edge Control VLAN is where that happens. Control traffic on the Fabric Control VLAN is never sent out of edge ports and is used for internal VCS control frames.

Now that we have a basic understanding of how VCS-enabled switches establish a Fabric we will close this post out. In the next VCS post we will discuss Layer 2 forwarding.