Concept Guide

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 congured values for the congurations 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 congured
parameter value be the same for the congurations in order to be compatible. For example, PFC uses an
symmetric exchange of parameters between DCBx peers.
Conguration Source Election
When an auto-upstream or auto-downstream port receives a DCB conguration from a peer, the port rst checks to see if there is an
active conguration source on the switch.
• If a conguration source already exists, the received peer conguration is checked against the local port conguration. If the received
conguration is compatible, the DCBx marks the port as DCBx-enabled. If the conguration 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 conguration is later received, DCBx is enabled on the
port.
• If there is no conguration source, a port may elect itself as the conguration source. A port may become the conguration source if
the following conditions exist:
– No other port is the conguration 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 conguration values through either a symmetric or asymmetric parameter
exchange.
A newly elected conguration source propagates conguration changes received from a peer to the other auto-conguration ports. Ports
receiving auto-conguration information from the conguration source ignore their current settings and use the conguration source
information.
Propagation of DCB Information
When an auto-upstream or auto-downstream port receives a DCB conguration from a peer, the port acts as a DCBx client and checks if a
DCBx conguration source exists on the switch.
• If a conguration source is found, the received conguration is checked against the currently congured values that are internally
propagated by the conguration source. If the local conguration is compatible with the received conguration, the port is enabled for
DCBx operation and synchronization.
• If the conguration received from the peer is not compatible with the internally propagated conguration used by the conguration
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 conguration is later received from the peer, the port is
enabled for DCBx operation.
NOTE
: DCB congurations internally propagated from a conguration source do not overwrite the conguration on a DCBx port
in a manual role. When a conguration source is elected, all auto-upstream ports other than the conguration source are marked
as willing disabled . The internally propagated DCB conguration is refreshed on all auto-conguration ports and each port may
begin conguration negotiation with a DCBx peer again.
298 Data Center Bridging (DCB)