NOTE: On a DCBx port, application priority TLV advertisements are handled as follows:• The application priority TLV is transmitted only if the priorities in the advertisement match the configured PFC priorities on theport.• On auto-upstream and auto-downstream ports:• If a configuration source is elected, the ports send an application priority TLV based on the application priority TLV received onthe configuration-source port. When an application priority TLV is received on the configuration-source port, the auto-upstreamand 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 configuredPFC 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 theport.DCB Configuration ExchangeThe DCBx protocol supports the exchange and propagation of configuration information for the enhanced transmission selection (ETS) andpriority-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 portand the local port use the same configured values for the configurations to be compatible. For example, ETS usesan 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 configuredparameter value be the same for the configurations in order to be compatible. For example, PFC uses ansymmetric exchange of parameters between DCBx peers.Configuration Source ElectionWhen an auto-upstream or auto-downstream port receives a DCB configuration from a peer, the port first checks to see if there is anactive configuration source on the switch.• If a configuration source already exists, the received peer configuration is checked against the local port configuration. If the receivedconfiguration is compatible, the DCBx marks the port as DCBx-enabled. If the configuration received from the peer is not compatible, awarning message is logged and the DCBx frame error counter is incremented. Although DCBx is operationally disabled, the port keepsthe peer link up and continues to exchange DCBx packets. If a compatible peer configuration is later received, DCBx is enabled on theport.• If there is no configuration source, a port may elect itself as the configuration source. A port may become the configuration source ifthe 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 parameterexchange.A newly elected configuration source propagates configuration changes received from a peer to the other auto-configuration ports. Portsreceiving auto-configuration information from the configuration source ignore their current settings and use the configuration sourceinformation.Data Center Bridging (DCB) 255
