Allied Telesis AT-S62 - Manual (Page 259)

AT-S62 Menus Interface User’s Guide
Section II: Advanced Operations 259
DiffServ domains.
❑ The DSCP value of the IP header’s TOS byte (Figure 66 on page
223). Replacing this field may be required as part of the
configuration of a DiffServ domain. See DiffServ Domains on page
259 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.