Flow Control
BCM5718 Programmer’s Guide
Broadcom
®
January 29, 2016 • 5718-PG108-R
Page 574
A p p e n d i x A : F l o w C o n t r o l
Notes
Developers can refer to the IEEE 802.3 Annex 31B specification for detailed information on Ethernet flow control
mechanisms.
• Flow control frames use a well-known multicast address, defined in the 802.1D Bridging specification The
MAC destination address is 01-80-C2-00-00-01.
• Bridges and Switches will not forward pause frames to downstream ports.
• A pause frame contains a request_operand that contains a pause_time field. Pause_time specifies the
number of quanta, which transmission should be inhibited.
• Pause frames cannot inhibit MAC control Frames
• Pause_time is a two-octet field, which represents a quanta value. The quanta value is based on bit/slot
times for the connection speed. Valid pause_times vary from 0 to 65535.
• The pause frame contains a MAC control opcode. 00-01 is reserved for PAUSE MAC control functions.
• MAC control layers will provide two indicators—paused and not paused.
• The Enet source address equals the unicast address of the MAC sublayer, which transmits the
pause_frame.
• The receive engine will set a countdown timer, based on the value of pause_time. When the timer expires,
the transmit engine may resume send operation
• A Mac sublayer may transmit pause frames with pause_time = 0. The zero value will stop a pause count
down, executed by the MAC’s link partner. Effectively, a value of zero restarts a link partner’s transmit
engine, assuming the link partner was inhibited by a previous pause operation.
Flow Control Scenario
This scenario assumes that the Gigabit switch has a 1:1 port mapping, between the Gigabit Server and Client.
The switch does not implement header aligned blocking, nor will it drop packets to alleviate buffer pressure. The
following constraints are placed on this scenario:
• Client
– Full-duplex connection at Gigabit speed.
– Implements flow control.
– Flow control enabled.
• Switch
– Does not drop packets.
– Full-duplex connection to Client.
• Server
– Gigabit connection.