Users Guide

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 congured PFC priorities on
the port.
• On auto-upstream and auto-downstream ports:
– If a conguration source is elected, the ports send an application priority TLV based on the application priority TLV
received on the conguration-source port. When an application priority TLV is received on the conguration-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 congured PFC priorities in application priority
TLVs.
– If no conguration source is congured, auto-upstream and auto-downstream ports check to see that the locally
congured 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
congured on the port.
DCB Conguration Exchange
On an Aggregator, the DCBx protocol supports the exchange and propagation of conguration information for the following DCB
features.
• Enhanced transmission selection (ETS)
• Priority-based ow control (PFC)
DCBx uses the following methods to exchange DCB conguration 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 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.
46
Data Center Bridging (DCB)