Allied Telesis AT-S62 - Manual (Page 273)

AT-S62 Management Software Menus Interface User’s Guide
Section II: Advanced Operations 273
Replacing
Priorities
The traffic class or flow group priority (if set) determines the egress queue
a packet is sent to when it egresses this switch, but by default has no
effect on how the rest of the network processes the packet. To
permanently change the packet’s priority, you need to replace one of two
priority fields in the packet header:
ˆ The User Priority field of the VLAN tag header. Replacing this field
relabels VLAN-tagged traffic, so that downstream switches can
process it appropriately. Replacing this field is most useful outside
DiffServ domains.
ˆ The DSCP value of the IP header’s TOS byte (Figure 66 on page 237).
Replacing this field may be required as part of the configuration of a
DiffServ domain. See “DiffServ Domains” on page 273 for information
on using the QoS policy model and the DSCP value to configure a
DiffServ domain.
VLAN Tag User
Priorities
Within a flow group or traffic class, the VLAN tag User Priority value of
incoming packets can be replaced with the priority specified in the flow
group or traffic class. Replacement occurs before the packet is queued, so
this priority also sets the queue priority.
DSCP Values
There are three methods of replacing the DSCP byte of an incoming
packet. You can use these methods together or separately. They are
described in the order in which the switch performs them.
1. The DSCP value can be overwritten at ingress, for all traffic in a policy.
2. The DSCP value in the packet can be replaced at the traffic class or
flow group level.
You can use these two replacements together at the edge of a DiffServ
domain, to initialize incoming traffic.
3. The DSCP value in a flow of packets can replaced if the bandwidth
allocated to that traffic class is exceeded, using the command. This
option allows the next switch in the network to identify traffic that
exceeded the bandwidth allocation.
DiffServ Domains
Differentiated Services (DiffServ) is a method of dividing IP traffic into
classes of service, without requiring that every router in a network
remember detailed information about traffic flows. DiffServ operates within
a DiffServ domain, a network or subnet is managed as a single QoS unit.
Packets are classified according to user-specified criteria at the edge of
the network, divided into classes, and assigned the required class of
service. Then packets are marked with a Differentiated Services Code
Point (DSCP) tag to indicate the class of service to which they belong. The
DSCP value is written into the TOS field of the IP header. Routers within
the network then use this DSCP value to classify packets, and assign QoS
appropriately. When a packet leaves the DiffServ domain, the DSCP value
can be replaced with a value appropriate for the next DiffServ domain.