Alcatel-Lucent 7450 Quality Of Service Manual page 244

Service SAP QoS Policy Command Reference
Description This command explicitly sets the forwarding class or subclass or enqueuing priority when a packet is
marked with the DiffServ Code Point (DSCP) value contained in the dscp-name. A list of up to 8 dscp-
names can be entered on a single command. The lists of dscp-names within the configuration are
managed by the system to ensure that each list does not exceed 8 names. Entering more than 8 dscp-
names with the same parameters (fc, priority) will result in multiple lists being created. Conversely,
multiple lists with the same parameters (fc, priority) are merged and the lists repacked to a maximum
of 8 per list if dscp-names are removed or the parameters changed so the multiple lists use the same
parameters. Also, if a subset of a list is entered with different parameters then a new list will be created
for the subset. When the list is stored in the configuration, the dscp-names are sorted by their DSCP
value in ascending numerical order, consequently the order in the configuration may not be exactly
what the user entered.
Adding a DSCP rule on the policy forces packets that match the DSCP value specified to override the
forwarding class and enqueuing priority based on the parameters included in the DSCP rule. When the
forwarding class is not specified in the rule, a matching packet preserves (or inherits) the existing
forwarding class derived from earlier matches in the classification hierarchy. When the enqueuing
priority is not specified in the rule, a matching packet preserves (or inherits) the existing enqueuing
priority derived from earlier matches in the classification hierarchy.
The DSCP value (referred to here by dscp-name) is derived from the most significant six bits in the
IPv4 header ToS byte field (DSCP bits) or the Traffic Class field from the IPv6 header. If the packet
does not have an IP header, dscp based matching is not performed. The six DSCP bits define 64 DSCP
values used to map packets to per-hop Quality-of-Service (QoS) behavior. The most significant three
bits in the IP header ToS byte field are also commonly used in a more traditional manner to specify an
IP precedence value, causing an overlap between the precedence space and the DSCP space. Both IP
precedence and DSCP classification rules are supported.
DSCP rules have a higher match priority than IP precedence rules and where a dscp-name DSCP value
overlaps an ip-prec-value, the DSCP rule takes precedence.
The no form of the command removes the specified the dscp-names from the explicit DSCP
classification rule in the SAP ingress policy. As dscp-names are removed, the system repacks the lists
of dscp-names with the same parameters (up to 8 per list). As the no command does not have any
additional parameters, it is possible to remove multiple dscp-names from multiple DSCP statements
having different parameters with one command. If a dscp-name specified in a no command does not
exist in any DSCP statement, then the command is aborted at that point with an error message
displayed; any dscp-names in the list before the failed entry will be processed as normal but the
processing will stop at the failed entry so that the remainder of the list is not processed.
Removing the dscp-name from the policy immediately removes the dscp-name on all ingress SAPs
using the policy.
This is supported on FP2- and higher-based line cards for the 7450 ESS.
Parameters dscp-name — The DSCP name is a required parameter that specifies the unique IP header ToS
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byte DSCP bits value that will match the DSCP rule. If the command is executed multiple
times with the same dscp-name, the previous forwarding class and enqueuing priority is
completely overridden by the new parameters or defined to be inherited when a forwarding
class or enqueuing priority parameter is missing.
Quality of Service Guide