User guide

Packet Over SONET Module Installation and User Guide 3-49
Configuring QoS Functions
The optional low-drop-probability and high-drop-probability keywords apply
only to PoS ports. If you do not enter either of these keywords in the command, the
command uses
low-drop-probability as the default.
The
low-drop-probability and high-drop-probability keywords are useful in
conjunction with the Weighted RED (WRED) implementation provided by PoS ports.
This implementation supports two different drop probabilities: one for DiffServ code
points designated as having low drop-probability; another for DiffServ code points
designated as having high drop-probability. These keywords give you complete
flexibility in assigning DiffServ code points to these two drop-probability levels.
Configuring DiffServ Code Point Mapping Tables
You can use the diffserv dscp-mapping command to configure a mapped relationship
between an input DSCP and an associated output DSCP. Each PoS port supports three
DSCP mapping tables: one of the tables is used in the ingress direction; two are used for
egress flows (onto the SONET link). The two egress tables are for the congested and
noncongested states, as determined by the RED algorithm. If RED is not enabled on the
PoS port, the egress congested-state mapping table is not used.
In the ingress direction, the input DSCP of a packet received from the SONET link is
replaced by an output DSCP before the packet is forwarded. In the egress direction, the
operation is similar, except that the DSCP mapping occurs before the packet is
transmitted onto the SONET link.
One potential use of the DSCP mapping capability is to reconcile varying DiffServ
policies at the boundary between autonomous systems, such as at the boundary
between two ISPs. The availability of different tables for the congested and
noncongested states is useful in marking operations that increase the probability of
packets being dropped during times of congestion, as discussed in the DiffServ Assured
Forwarding RFC (RFC 2597).
This command applies only to PoS ports with IP frames that are encapsulated in
BCP or IPCP, not MLSCP. You should also be aware that DSCP mapping is
performed even when the
diffserv examination function is disabled on the
port.
To configure the mapping between an input DSCP and an associated output DSCP, use
the following command:
config diffserv dscp-mapping <input_codepoint>/<output_codepoint>
ports <portlist> {egress {no-congestion | congestion} | ingress}