Here's some data on pause.
It is a method of flow control which opperates at the MAC layer. Be aware there is a danger if using pause and TCP that the two methods of flow control running simultaneously can cause problems.
If a buffer on a Gigabit Ethernet port runs out of space, the port transmits a special packet that requests remote ports to delay sending packets for a period of time. This special packet is called a pause frame.
All Catalyst Gigabit Ethernet ports can receive and process pause frames from other devices. However, not all Catalyst Gigabit Ethernet ports can transmit pause frames to other devices.
802.3 Annex 31B
Method of temporarily stopping communication
The overwhelmed network element will send a PAUSE frame, which halts the transmission of the sender for a specified period of time.
PAUSE frames permit one end station to temporarily stop all traffic from the other end station
Works with full duplex only
PAUSE frame specifies how long pause should be expressed in units of 512 bit times
If an additional PAUSE frame arrives before the current PAUSE time has expired, its parameter replaces the current PAUSE time, so a PAUSE frame with parameter zero allows traffic to resume immediately.
New MAC control frame for the purpose of per priority or class based PAUSE
Used to control immediate link partner, not the connection partner
Support of PAUSE is optional
Destination Address 01-80-C2-00-00-01
CBFC class based flow control
It is valid to only support half the protocol (just send, or just receive PAUSE)
TX send and RX receive PAUSE parameters are negotiated separately
Other ends PAUSE capabilities negotiated with speed and duplex
Pause does not prioritize which protocols to refuse, it pauses ALL traffic
All Cisco Catalyst GbE ports can receive and process pause frames but not all can transmit them
Can cause head-of-line blocking, which messes with TCP flow control
Some Cisco switches are even permanently configured to receive pause frames but never emit them.
