DCB Configuration Exchange
The DCBx protocol supports the exchange and propagation of configuration information for the
enhanced transmission selection (ETS) and priority-based flow control (PFC) DCB features.
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 but
requires that each configured parameter value be the same for the configurations
in order 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.
• If a configuration source is found, the received configuration is checked against the currently
configured values that are internally propagated by the configuration source. If the local configuration
is compatible with the received configuration, the port is enabled for DCBx operation and
synchronization.
276 Data Center Bridging (DCB)