Administrator Guide

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.
• If the configuration received from the peer is not compatible with the internally propagated configuration used by the configuration
source, the port is disabled as a client for DCBx operation and synchronization and a syslog error message is generated. The port
keeps the peer link up and continues to exchange DCBx packets. If a compatible configuration is later received from the peer, the port
is enabled for DCBx operation.
NOTE:
DCB configurations internally propagated from a configuration source do not overwrite the configuration on a
DCBx port in a manual role. When a configuration source is elected, all auto-upstream ports other than the configuration
source are marked as willing disabled . The internally propagated DCB configuration is refreshed on all auto-
configuration ports and each port may begin configuration negotiation with a DCBx peer again.
250 Data Center Bridging (DCB)