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On a DCBx port in an auto-upstream role, the PFC and application priority TLVs are enabled. ETS
recommend TLVs are disabled and ETS configuration TLVs are enabled.
Auto-
downstream
The port advertises its own configuration to DCBx peers but is not willing to receive remote peer
configuration. The port always accepts internally propagated configurations from a configuration source.
An auto-downstream port that receives an internally propagated configuration overwrites its local
configuration with the new parameter values.
When an auto-downstream port receives and overwrites its configuration with internally propagated
information, one of the following actions is taken:
● If the peer configuration received is compatible with the internally propagated port configuration, the
link with the DCBx peer is enabled.
● If the received peer configuration is not compatible with the currently configured port configuration,
the link with the DCBx peer port is disabled and a syslog message for an incompatible configuration
is generated. The network administrator must then reconfigure the peer device so that it advertises a
compatible DCB configuration.
The internally propagated configuration is not stored in the switch’s running configuration. On a DCBx
port in an auto-downstream role, all PFC, application priority, ETS recommend, and ETS configuration
TLVs are enabled.
Default DCBx port role: Uplink ports are auto-configured in an auto-upstream role. Server-facing ports are auto-configured in
an auto-downstream role.
NOTE: You can change the port roles only in the PMUX mode. Use the following command to change the port roles:
dcbx port-role {auto-downstream | auto-upstream | config-source | manual}
manual is the default port role.
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 the port.
● 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 on the configuration-source port. When an application priority TLV is received on the configuration-source
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
On an Aggregator, 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 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.
238 Data Center Bridging (DCB)