Service Egress and Ingress QoS Policies
Quality of Service Guide 275
When collect-stats is enabled, the counters are used by the system to generate the following
statistics:
a. ‘offered-in’ = 1 + 2
b.‘offered-out’ = 3 + 4
c.‘discard-in’ = 5 + 7
d.‘discard-out’ = 6 + 8
e.‘forward-in’ = 9 + 11
f.‘forward-out’ = 10 + 12
prec
Syntax prec ip-prec-value fc fc-name [priority {high | low}]
no prec ip-prec-value
Context config>qos>sap-ingress
Description This command explicitly sets the forwarding class or enqueuing priority when a packet is marked with
an IP precedence value (ip-prec-value). Adding an IP precedence rule on the policy forces packets that
match the specified ip-prec-value to override the forwarding class and enqueuing priority based on the
parameters included in the IP precedence 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 ip-prec-value is derived from the most significant three bits in the IP header ToS byte field
(precedence bits). The three precedence bits define 8 Class-of-Service (CoS) values commonly used
to map packets to per-hop Quality-of-Service (QoS) behavior. The precedence bits are also part of the
newer DiffServ Code Point (DSCP) method of mapping packets to QoS behavior. The DSCP uses the
most significant six bits in the IP header ToS byte and so overlaps with the precedence bits. 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 HSMDA queue offered stats represent all packets sent to a specific ingress or egress queue
regardless of the HSMDA counter override. This results in accurate queue offered stats, while the
discard and forwarding stats per queue only represent packets that have not been associated with an
exception counter. If the queue discard and forwarding stats are subtracted from the queue offered
stats, an approximation of the number of packets handled by the queue that have been associated with
an exception counter may be calculated. This is an approximation due to the possible presence of
packets currently in the queue that are not represented by the discard or forwarding stats at the time the
stats are collected but had been included in the queue offered stats. This discrepancy is minimized
when the stats are collected over time and disappears completely once the queue drains.
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