Page 6 | AlliedWare Plus™ OS: Overview of QoS
Classification
Classification is simply a method of dividing the incoming traffic into traffic flows so that
packets of one type can be treated differently to packets of another type. To do this, you
create class maps and if desired ACLs. Incoming packets are inspected and may be classified
on a very broad range of criteria.
The classification process does not update any of the four marker values on the packet, but
does dictate the path that the packet will subsequently take through the QoS system.
Premarking
The “pre” part of premarking means this process happens before any bandwidth policing
takes place. The “marking” part refers to attaching QoS information to packets.
One possible use for this is to apply a DSCP value to a traffic stream. For example, packets
coming from a database server could require assured forwarding treatment, and so could be
marked with DSCP=
1
8 at ingress to the switch.
Recall that packets can be marked in four ways:
zthe VLAN tag user priority
zthe Differentiated Services Code Point (DSCP)
zthe bandwidth class the packet is assigned to
zthe egress queue the packet is assigned to.
A packet can have new values assigned for each of these marking values by the premarking
process. There are two mutually exclusive methods available for premarking:
zsetting the new values explicitly for all packets that match a certain class map, or
zlooking up the mark-dscp map and applying the map’s values to the packets. The mark-dscp
map is a user-defined table that maps particular DSCP values to particular sets of 802.
1
p,
DSCP, bandwidth class, and egress queue values. See "Premarking"on page 13 for a tab le
that shows the mark-dscp map structure.
If premarking uses the mark-dscp map, there are two ways to choose the DSCP value to use
in looking up entries in the mark-dscp map:
zuse the existing DSCP value of the packet (different packets within the class map may well
have different DSCP values)
zspecify a single DSCP value that QoS will use for look-ups for all packets that match the
class map.
Whichever of these two criteria is used, the value is used to index the mark-dscp map.