Brocade Multi-Service IronWare QoS and Traffic Management Configuration Guide (Supporting R05.6.00) - Manual (Page 56)

42 Multi-Service IronWare QoS and Traffic Management Configuration Guide
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Ingress Traffic processing through a device
3
Traffic types
Data - Data packets can be either Network-to-Network traffic or traffic from the CPU.
Network-to-Network traffic is considered Data traffic.Qos parameters can be assigned and
modified for data traffic.
Control - Packets to and from the CPU is considered control traffic. The QoS parameters fro this
traffic are preassigned and not configurable.
Setting packet header QoS fields
The device supports setting or modifying the packet header IEEE 802.1p User Priority or IP-DSCP.
Packet QoS attributes
Every packet classified as Data is assigned a set of QoS attributes that can be modified by each
ingress pipeline engine.
Each of the ingress pipeline engines contain several Initial QoS Markers that assign the packet’s
initial QoS attribute.
The ingress pipeline engine also contains a QoS Remarker that can modify the initial QoS
attributes.
Even though Brocade NetIron CES and Brocade NetIron CER devices support four drop precedence
values 0,1,2 and 3 internally 1 and 2 are assigned the same drop precedence level. The four levels
are kept for CLI compatibility with other Brocade devices. Three internal level of drop precedence
are 0, {1,2} and 3. in terms of commonly used color based terminology: 0 represents Green (lowest
drop precedence}, 1 and 2 represents yellow (higher drop precedence) and 3 represents Red
(highest drop precedence).
Ingress Traffic processing through a device
The QoS operation on Ingress traffic of a Brocade device involves reception and processing of
packets based upon priority information contained within the packet. As the packets are processed
through the device, there are several opportunities to influence the processing by configuration as
described in the following steps.
1. Derive priority and drop precedence from the packets PCP (802.1p) value. The Priority Code
Point (PCP) is a 3-bit field within an 802.1Q tagged frame that is used to convey the priority of
the frame. By using a mapping table, the 3-bit PCP field can be decoded to derive priority and
drop precedence information. Note: the PCP field was formerly called 802.1p.
2. Derive priority and drop precedence from the packets DSCP value.
TABLE 14 Packet QoS attributes
QoS parameter Description
TC (Traffic Class) This is the priority level assigned to the packet. When the TxQ enqueues
the packet, it uses this field to select the appropriate priority queue.
DP (Drop Precedence) The TxQ uses this field for congestion resolution. Packets with higher
drop precedence are more likely to be discarded in the event of
congestion.