Buffer allocation/threshold for qos queues, Marking, Configuring dscp-based qos – Brocade TurboIron 24X Series Configuration Guide User Manual
Page 1075: Marking 1, Configuring dscp-based qos 1
Brocade TurboIron 24X Series Configuration Guide
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Marking
Buffer allocation/threshold for QoS queues
By default, Ironware software allocates a certain number of buffers to the outbound transport
queue for each port based on QoS priority. The buffers control the total number of packets
permitted in the outbound queue for the port. If desired, you can increase or decrease the
maximum number of outbound transmit buffers allocated to all QoS queues, or to specific QoS
queues on a port or group of ports. For more information, refer to
“Egress buffer thresholds for QoS
on page 267. This feature is called egress buffer threshold. For more information, refer
to
“Egress buffer thresholds for QoS priorities”
Marking
Marking is the process of changing the packet QoS information (the 802.1p and DSCP information
in a packet) for the next hop. For example, for traffic coming from a device that does not support
DiffServ, you can change the packet IP Precedence value into a DSCP value before forwarding the
packet.
You can mark a packet Layer 2 CoS value, its Layer 3 DSCP value, or both values. The Layer 2 CoS
or DSCP value the device marks in the packet is the same value that results from mapping the
packet QoS value into a Layer 2 CoS or DSCP value.
Marking is optional and is disabled by default. Marking is performed using ACLs. When marking is
not used, the device still performs the mappings listed in
on page 1035for
scheduling the packet, but leaves the packet QoS values unchanged when the device forwards the
packet.
For configuration syntax, rules, and examples of QoS marking, refer to
Configuring DSCP-based QoS
IronWare releases support basic DSCP-based QoS (also called Type of Service (ToS) based QoS) as
described in this chapter. However, the TurboIron X Series family of switches do not support other
advanced DSCP-based QoS features as described.
IronWare releases also support marking of the DSCP value. The software can read Layer 3 Quality
of Service (QoS) information in an IP packet and select a forwarding queue for the packet based on
the information. The software interprets the value in the six most significant bits of the IP packet
header 8-bit ToS field as a Diffserv Control Point (DSCP) value, and maps that value to an internal
forwarding priority.
The internal forwarding priorities are mapped to one of the eight forwarding queues (qosp0 –
qosp7) on the device. During a forwarding cycle, the device gives more preference to the higher
numbered queues, so that more packets are forwarded from these queues. So for example, queue
qosp7 receives the highest preference while queue qosp0, the best-effort queue, receives the
lowest preference.