Chapter 21
| Data Center Bridging Commands
Priority-based Flow Control Commands
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Default Setting
Shows DCBX configuration settings for all ports.
Command Mode
Privileged Exec
Example
This example displays the DCBX administrative status, operational mode, and the
status of the LLDP TLV willing bit for ETS and PFC.
Console#show dcbx ethernet 1/5
DCBX Port Configuration
Port Status Mode ETS Willing PFC Willing
-------- -------- ------------------- ----------- -----------
Eth 1/5 Enabled AutoUpstream No No
Console#
Priority-based Flow Control Commands
Priority-based Flow Control (PFC) is used to reduce frame loss due to congestion by
inhibiting the transmission of frames based on individual traffic classes. PFC can
pause high priority traffic only when necessary to avoid dropping frames, while
allowing traditional traffic assigned other priorities to continue flowing through an
interface.
Traffic classes are specified in the priority field of the 802.1Q VLAN header, which
identifies an 802.1p priority value. However, a VLAN unaware end station can also
use PFC by sending traffic as priority-tagged and ignoring the VLAN ID in received
frames. Note that some frames, such as BPDUs, are sent untagged and can bypass
the output queues, it is strongly recommended that the default priority for a port
not have PFC enabled.
PFC can reduce the number of frames discarded due to congestion for loss-
sensitive protocols. However, PFC can cause congestion to spread, and is therefore
intended for use on networks of limited extent, such as within a data center. When
PFC is used, deployment of Congestion Notification (CN) can reduce the frequency
at which PFC is invoked.
Table 110: Priority-based Flow Control Commands
Command Function Mode
pfc mode Sets the PFC mode to negotiate capability through
DCBX or by forcing it to on state
IC
pfc priority Enables PFC for specified priorities IC
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