Nortel business policy switch 2000 User Manual page 297

The Business Policy Switch does not trust the DSCP of IP traffic received from an
untrusted port, but it does trust the DSCP of IP traffic received from a trusted port.
Filters installed on trusted ports cannot change the DSCP of the IP packets
received on these ports. These filters specify an action that must change the IEEE
802.1p and drop precedence of the matching packets based on the incoming DSCP
using a table that matches each one of the 64 DSCP values to the corresponding
IEEE 802.1p priority. The values can be modified by a policy server or by the user.
If a packet is received from a trusted port and either it does not match any of the
filters installed by the user on this port or it does match a filter but is not dropped,
the BPS 2000 uses a default layer 2 filter to change the packet IEEE 802.1p and
drop precedence based on the DSCP of the packet.
Filters that you install on untrusted ports must specify an action to change the
DSCP, IEEE 802.1p priority, and drop precedence of IP traffic received from these
ports. For non-IP traffic, the filters must specify an action to update the IEEE
802.1p priority and drop precedence, but not update the DSCP.
If an IP packet is received from an untrusted port and it does not match any one of
the filters installed by the user on the port, the BPS 2000 uses default layer 2
filters to change the packet DSCP, IEEE 802.1p priority, and drop precedence as
follows:
• If the packet is tagged, the BPS 2000 uses a layer 2 filter to change the DSCP,
IEEE 802.1p to 0, and drop precedence to 1 so that the packet can get best
effort treatment.
• If an IP packet is untagged, the BPS 2000 uses 8 default layer 2 filters to
change the DSCP based on the default IEEE 802.1p priority of the ingress
untrusted port. The BPS 2000 changes the packet DSCP using the 802.1p
priority mapping table that matches each one of the eight IEEE 802.1p
priorities to the corresponding DSCP. The values can be modified by a policy
server or by the user.
The unrestricted ports, or the unrestricted class of interface groups, have no
restrictions. That is, you can re-mark the DSCP or not, depending on your
configuration. Using unrestricted ports allows you to manipulate the DSCP value
based on the filter criteria.
Table 59 describes the default DSCP, QoS class, IEEE 802.1p, and egress queue
assignment for packets in each traffic class.
Chapter 4 Policy-enabled networks 297
Using the Business Policy Switch 2000 Version 2.0