Network Router User Manual

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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 42 Configuring PFC QoS
PFC QoS Glossary
PFC QoS Glossary
This section defines some of the QoS terminology used in this chapter:
• Buffers—A storage area used for handling data in transit. Buffers are used in internetworking to
compensate for differences in processing speed between network devices. Bursts of data can be stored in
buffers until they can be handled by slower processing devices. Sometimes referred to as a packet buffer.
• Class of Service (CoS) is a Layer 2 QoS label carried in three bits of either an ISL, 802.1Q, or 802.1p
header. CoS values range between zero and seven.
• Classification is the process used for selecting traffic to be marked for QoS.
• Congestion avoidance is the process by which PFC QoS reserves ingress and egress LAN port
capacity for Layer 2 frames with high-priority Layer 2 CoS values. PFC QoS implements
congestion avoidance with Layer 2 CoS value-based drop thresholds. A drop threshold is the
percentage of queue buffer utilization above which frames with a specified Layer 2 CoS value is
dropped, leaving the buffer available for frames with higher-priority Layer 2 CoS values.
• Differentiated Services Code Point (DSCP) is a Layer 3 QoS label carried in the six most-significant
bits of the ToS byte in the IP header. DSCP ranges between 0 and 63.
• Frames carry traffic at Layer 2. Layer 2 frames carry Layer 3 packets.
• IP Precedence is a Layer 3 QoS label carried in the three most-significant bits of the ToS byte in the
IP header. IP precedence ranges between zero and seven.
• Labels—See QoS labels.
144803
3 bits used for CoS
Layer 2 ISL frame
ISL header
(26 bytes)
Encapsulated frame 1...
(24.5 KB)
FCS
(4 bytes)
Layer 2 802.1Q and 802.1p frame
Preamble
Start frame
delimiter
DA SA Ta g PT Data FCS
3 bits used for CoS (user priority)
144804
Len
Layer 3 IPv4 packet
Version
length
To S
(1 byte)
ID Offset TTL Proto FCS IP-SA IP-DA Data
3 bits for IP precedence
144804
Len
Layer 3 IPv4 packet
Version
length
To S
(1 byte)
ID Offset TTL Proto FCS IP-SA IP-DA Data
3 bits for IP precedence