Reference Guide

port, the auto-upstream and auto-downstream ports use the internally propagated PFC priorities to match against
the received application priority. Otherwise, these ports use their locally configured PFC priorities in application
priority TLVs.
○ If no configuration source is configured, auto-upstream and auto-downstream ports check to see that the locally
configured PFC priorities match the priorities in a received application priority TLV.
● On manual ports, an application priority TLV is advertised only if the priorities in the TLV match the PFC priorities
configured on the port.
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.
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Data Center Bridging (DCB)