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QoS
Figure 27-1
Switch 1
Edge quench congestion control provides feedback to the source about the rate at which frames should
be injected into the network (frame intervals).
FCC Process
When a node in the network detects a congestion for an output port, it generates an edge quench message.
These frames are identified by the Fibre Channel destination ID (DID) and the source ID. A switch from
other vendors simply forwards these frames.
Any receiving switch in the Cisco MDS 9000 Family handles frames in one of these ways:
•
•
The behavior of the flow control mechanism differs based on the Fibre Channel DID:
•
•
•
All switches (including the edge switch) along the congested path process path quest frames. However,
only the edge switch processes edge quest frames.
Enabling FCC
By default, the FCC protocol is disabled. FCC can only be enabled for the entire switch.
If you enable FCC, be sure to enable it in all switches in the fabric.
QoS
QoS implementation in the Cisco MDS 9000 Family follows the Differentiated Services (DiffServ)
model. The DiffServ standard is defined in RFCs 2474 and 2475.
Cisco MDS 9000 Fabric Manager Switch Configuration Guide
27-2
FCC Mechanisms
Switch 1 sends
regular traffic to
Switch 2
It forwards the frame.
It limits the rate of the frame flow in the congested port.
If the Fibre Channel DID is directly connected to one of the switch ports, the input rate limit is
applied to that port.
If the destination of the edge quest frame is a Cisco domain or the next hop is a Cisco MDS 9000
Family switch, the frame is forwarded.
If neither of these mechanisms is true, then the frame is processed in the port going towards the FC
DID.
Switch 2 sends
congested traffic
to Switch 3
Switch 2
Chapter 27
Configuring Traffic Management
Switch 3 sends
congestion control
message to Switch 1
to slow down the
traffic control
Switch 3
OL-7753-01
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