Allied Telesis AT-8100 Series - Manual (Page 1780)

Chapter 101: QoS Commands
1780
TRUST DSCP
Syntax
trust dscp
Parameters
None
Mode
Policy Map Class mode
Description
Use this command to trust the DSCP field in an IP packet header to
prioritize and preserve their prioritization values in Layer 3 packets. You
set this command within a policy map, thereby enabling the policy map to
trust DSCP frames. By default, the DSCP field in Layer 3 packets is
ignored by the software. This command consults the DSCP-queue map,
configured with the MLS QOS MAP DSCP-QUEUE command, to prioritize
ingress traffic. See “MLS QOS MAP DSCP-QUEUE” on page 1740.
When you set a port to trust DSCP frames, the CoS value in the VLAN tag
field is re-marked. For example, using the default DSCP settings in
Table 210 on page 1661, a trust DSCP value of 46 on the ingress port
causes it to egress on queue 5. As a result, the CoS frame will be re-
marked to 5. This switch behavior exists so a packet carries both Layer 2
CoS packets and Layer 3 DSCP frames as it is passes downstream
through the network.
Note
If the frame is not set to egress as a tagged frame, the CoS value is
not an issue, because in this case, the entire VLAN tag is stripped
off the frame.
Use the NO TRUST DSCP command to remove the DSCP trust setting
from the specified policy map.
Examples
This example enables the DSCP-queue map lookup for the prioritization of
all traffic classified by a class map named “cmap1:”