DCBX Protocol – Cisco Describing Data Center Ethernet Enhancements

DCBX Protocol

The PFC and the ETS provide the means to divide, mark, and control the traffic and the bandwidth used, but there is still the need for these parameters and capabilities to be exchanged and negotiated between the devices in the converged infrastructure. The devices need to support the same standards. They need to be enabled. There is the need to exchange information about which CoS value is assigned to which traffic class. What is the priority for that class, and will the pause frame be used for it?

The converged, expanded Ethernet fabric consists of switches that support the DCB extensions to allow communication over the Ethernet infrastructure of both Ethernet and FCoE (or similar) traffic. Connected to the converged Ethernet infrastructure, also called a unified fabric, are end devices that support multiple different types of communication as well as non-DCB Ethernet switches. Because of that, two challenges need to be solved in such environments:

  • Define the borders of the unified fabric, or identify which devices, switches, and end devices support the converged infrastructure and which do not.
  • Exchange information about DCB support and capabilities between the devices in the unified fabric

To achieve that, the IEEE 802 working group specified the IEEE 802.1Qaz Data Center Bridging Exchange (DCBX) protocol (see Figure 14-5), which takes care of the exchange and negotiation of the following items:

  

Figure 14-5 IEEE 802.1Qaz DCBX

  • Traffic classes
  • Drop/no drop policy (PFC)
  • Priority groups in ETS
  • Logical link-down
  • Applications such as FCoE

The DCBX utilizes the IEEE 802.1AB LLDP as the transport protocol to communicate between the two devices. The DCBX communication consists of request and acknowledgment messages, as the information is encoded as TLV parameters.

When the devices, switches, and CNAs are connected to each other and the interfaces are enabled, the Cisco switches immediately try to initiate DCBX communication with the other devices. In case the other devices also support the DCB, the DCBX negotiations continue. In the LLDP TLVs, the parameters for the PFC, ETS, and the rest of the required information are exchanged to create a lossless environment for communication.