DCBX is defined in IEEE 802.1Qaz. DCBX is a discovery and capability exchange protocol that IEEE DCBs use to discover peers and exchange configuration information between DCB-compliant bridges (see Figure 9-6). The following parameters can be exchanged with DCBX:
Priority groups in ETS
PFC
Congestion notification
Applications
Logical link-down
Network interface virtualization
Figure 9-6 Data Center Bridging Exchange
DCBX allows network devices to advertise their identities and capabilities over the network. It enables end devices to pick up proper configuration from the network and for switches to verify proper configuration.
The DCBX protocol is an extension of the Link Layer Discovery Protocol (LLDP). DCBX endpoints exchange request and acknowledgment messages. For flexibility, parameters are coded in a type-length-value (TLV) format. DCBX runs on the physical Ethernet link between the Cisco Nexus device and the CNA. By default, DCBX is enabled on Ethernet interfaces. When an Ethernet interface is brought up, the switch automatically starts to communicate with the CNA. During the normal operation of FCoE between the switch and the CNA, DCBX provides link-error detection. DCBX is also used to negotiate capabilities between the switch and the CNA and to send configuration values to the CNA.
The CNAs that are connected to a Cisco Nexus device are programmed to accept the configuration values sent by the switch, allowing the switch to distribute configuration values to all attached CNAs. This reduces the possibility of configuration errors and simplifies CNA administration.
Data Center Bridging Exchange Protocol (DCBXP) is enabled by default when you enable LLDP. When LLDP is enabled, DCBXP can be enabled or disabled using the [no] lldp tlv-select dcbxp command. DCBXP is disabled on ports where LLDP transmit or receive is disabled.
You can enable LLDP on each FCoE switch by issuing the feature lldp command. On the Cisco Nexus 7000, LLDP is enabled when the FCoE feature set is installed (in the storage VDC). You cannot disable LLDP while the FCoE feature is installed.
The switch and CNA exchange capability information and configuration values. Cisco Nexus devices support the following capabilities:
FCoE: If the CNA supports FCoE capability, the switch sends the IEEE 802.1p CoS value to be used with FCoE packets.
PFC: If the adapter supports PFC, the switch sends the IEEE 802.1p CoS values to be enabled with PFC.
Priority group type, length, and values (TLV)
Ethernet logical link up and down signal
FCoE logical link up and down signal for pre-FIP CNAs
The following rules determine whether the negotiation results in a capability being enabled: