312 Chapter 8: QoS Support on the Catalyst 6500
The second policing action adheres to traffic exceeding the normal rate, but conforming to
the excess rate. In Hybrid mode, packets exceeding the normal rate, but conforming to the
excess, rate are marked down based on the normal rate policed DSCP table. In Native mode,
the administrator can instruct the policing logic to drop, mark down, or forward traffic
matching the criteria for the second policing action. In addition, in Native mode, the per-
port aggregate policer also allows a new DSCP or IP precedence value to be assigned. The
drop keyword is not available in Hybrid for traffic conforming to the excess rate, but
violating the normal rate. In Native mode, however, if the drop keyword is specified, the
resulting behavior can be compared to the single-rate policer.
The third policing action defines the policy applied to all traffic violating the normal and
excess rates. For Hybrid and Native modes, packets can either be dropped or marked and
forwarded with a policed DSCP value. In addition, Native mode provides the transmit
keyword as an option for violating traffic. When defining the policing policies for the three
different policing levels, subsequent policing policies cannot be less stringent than previ-
ously defined policies. If the policed-dscp-transmit keyword is specified for the exceed-
action, for example, transmit cannot be specified for the violate-action. The configured
action must be equal or more severe.
With the release of CatOS 7.2 and later versions, the eburst parameter is configured
independently of the burst parameter. This provides additional autonomy when config-
uring the burst parameters. Prior to the 7.2 release, the administrator is limited to only
configuring the burst option. The eburst value is derived from the burst value. The burst
and eburst values define the bucket depth for the respective token buckets. The two token
buckets, controlled by the policing ASIC on the PFC, operate independently of each other.
Tokens configured for the normal bucket are only allocated to packets conforming to the
normal rate, whereas tokens configured for the excess bucket are only allocated to packets
conforming to the excess rate. Tokens from the two buckets cannot be combined to service
a packet. Enough tokens must be present in either bucket to service an entire packet. If the
optional eburst value is not explicitly configured, the burst size is set to the same value for
both normal and excess rate policers.
When configuring the two-rate policer, ensure the configured burst values are at least equal
to the average or largest serviced packet size. In addition, set the excess rate value to be
greater than or equal to the normal rate. If the two policing rates are configured equally, the
resulting behavior is a single-rate policer. In this situation, although there are two separate
token buckets, one for normal rate and the other for excess rate, both buckets are charged
for a successfully transmitted packet. Therefore, if a packet arrives, which does not exceed
the normal rate, and the normal rate equals the excess rate, tokens amounting to the size of
the packet are depleted from both buckets. The following examples demonstrate config-
uring an aggregate two-rate policer in Hybrid and Native modes.
Example 8-33 Configuring an Aggregate Two-Rate Policer in Hybrid Mode
hybrid (enable) set qos policer aggregate HTTP-police rate 1000 policed-dscp erate
2000 drop burst 13 eburst 13
QoS policer for aggregate HTTP-police updated successfully.
Rate is set to 992, erate is set to 1984 burst is set to 13 and eburst is set to 13
in hardware due to hardware granularity.