Reference Guide

Data Center Bridging (DCB) | 295
DCB Configuration Exchange
The DCBx protocol supports the exchange and propagation of configuration information for the following
DCB features.
• Enhanced transmission selection (ETS)
• Priority-based flow control (PFC)
DCBx uses the following methods to exchange DCB configuration parameters:
• Asymmetric: DCB parameters are exchanged between a DCBx-enabled port and a peer port without
requiring that a peer port and the local port use the same configured values for the configurations to be
compatible. For example, ETS uses an asymmetric exchange of parameters between DCBx peers.
• Symmetric: DCB parameters are exchanged between a DCBx-enabled port and a peer port with the
requirement that each configured parameter value is the same for the configurations to be compatible.
For example, PFC uses an symmetric exchange of parameters between DCBx peers.
Configuration Source Election
When an auto-upstream or auto-downstream port receives a DCB configuration from a peer, the port first
checks to see if there is an active configuration source on the switch.
• If a configuration source already exists, the received peer configuration is checked against the local
port configuration. If the received configuration is compatible, the DCBx marks the port as
DCBx-enabled. If the configuration received from the peer is not compatible, a warning message is
logged and the DCBx frame error counter is incremented. Although DCBx is operationally disabled,
the port keeps the peer link up and continues to exchange DCBx packets. If a compatible peer
configuration is later received, DCBx is enabled on the port.
• If there is no configuration source, a port may elect itself as the configuration source. A port may
become the configuration source if the following conditions exist:
• No other port is the configuration source.
• The port role is auto-upstream.
• The port is enabled with link up and DCBx enabled.
• The port has performed a DCBx exchange with a DCBx peer.
• The switch is capable of supporting the received DCB configuration values through either a
symmetric or asymmetric parameter exchange.
A newly elected configuration source propagates configuration changes received from a peer to the other
auto-configuration ports. Ports receiving auto-configuration information from the configuration source
ignore their current settings and use the configuration source information.
Propagation of DCB Information
When an auto-upstream or auto-downstream port receives a DCB configuration from a peer, the port acts
as a DCBx client and checks if a DCBx configuration source exists on the switch.