Quick Reference Guide

410 PowerConnect B-Series TI24X Configuration Guide
53-1002269-02
Marking
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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
priorities” on page 187. On PowerConnect B-Series TI24X devices, this feature is called egress
buffer threshold. For more information, refer to “Egress buffer thresholds for QoS priorities” on
page 187.
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 “Classification” on page 405for
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 “QoS options for IP ACLs” on
page 387.
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 PowerConnect B-Series TI24X 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.