Which functionality enables creating virtual links on a single physical link and independently
pausing a virtual link in an FCoE environment?

A.
Enhanced transmission selection

B.
Data center bridging exchange

C.
Buffer-to-buffer credit

D.
Priority-based flow control

Explanation:

Priority Based Flow Control

Traditional FC manages congestion through the use of a link-level, credit-based flow control that
guarantees no loss of frames. Typical Ethernet, coupled with TCP/IP, uses a packet drop flow
control mechanism. The packet drop flow control is not lossless. This challenge is eliminated by
using an IEEE 802.3x Ethernet PAUSE control frame to create a lossless Ethernet. A receiver can
send a PAUSE request to a sender when the receiver’s buffer is filling up. Upon receiving a

PAUSE frame, the sender stops transmitting frames, which guarantees no loss of frames. The
downside of using the Ethernet PAUSE frame is that it operates on the entire link, which might be
carrying multiple traffic flows.
PFC provides a link level flow control mechanism. PFC creates eight separate virtual links on a
single physical link and allows any of these links to be paused and restarted independently.
PFC enables the pause mechanism based on user priorities or classes of service. Enabling the
pause based on priority allows creating lossless links for traffic, such as FCoE traffic. This PAUSE
mechanism is typically implemented for FCoE while regular TCP/IP traffic continues to drop
frames. Figure in the slide illustrates how a physical Ethernet link is divided into eight virtual links
and allows a PAUSE for a single virtual link without affecting the traffic for the others.