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PowerConnect B-Series FCX Configuration Guide 599
53-1002266-01
Marking
17
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
Differentiated Services (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 591 for
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 579.
Configuring DSCP-based QoS
Dell IronWare releases support basic DSCP-based QoS (also called Type of Service (ToS)-based
QoS) as described in this chapter. However, the PowerConnect family of switches does not support
other advanced DSCP-based QoS features as described in the Enterprise Configuration and
Management Guide.
Dell 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 through
qosp7) on the Dell PowerConnect device. During a forwarding cycle, the device gives more
preference to the higher numbered queues, so that more packets are forwarded from these
queues. For example, queue qosp7 receives the highest preference while queue qosp0, the
best-effort queue, receives the lowest preference.
Application notes
• DSCP-based QoS is not automatically honored for routed and switched traffic. The default is
802.1p to CoS mapping. To honor DSCP-based QoS, you must either use ACL or enable trust
DSCP. Refer to “Using ACLs to honor DSCP-based QoS” on page 599.
• When DSCP marking is enabled, the device changes the contents of the inbound packet ToS
field to match the DSCP-based QoS value. This differs from BigIron, which marks the outbound
packet ToS field.
Using ACLs to honor DSCP-based QoS
This section shows how to configure Dell PowerConnect devices to honor DSCP-based QoS for
routed and switched traffic.