Cisco Catalyst X4232 Installation And Configuration Note page 71

Configuring Layer 3 QoS
A token bucket is a formal definition of a rate of transfer. The mean rate determines the long-term
average transmission rate, and the burst size determines how large traffic bursts can be sent before some
traffic exceeds the contract.
Tokens are inserted into the bucket at the mean rate. The depth of the bucket is the burst size. Traffic
arriving at the bucket when sufficient tokens are available is said to conform, and the corresponding
number of tokens are removed from the bucket. If sufficient tokens are not available, then the traffic is
said to be nonconforming.
The rate limiter and the shaper identify traffic violations in an identical manner. They differ, however,
in the way they respond to traffic violations. The rate limiter drops the traffic that is not conforming to
the traffic contract, whereas the shaper delays excess traffic using a buffer and drops traffic only when
the buffer overflows.
Token bucket-based per-port traffic conditioning features support a minimum rate of 32 kbps and have
a granularity of 32 kbps. The additional processing associated with these features may affect the peak
data forwarding performance that limits the peak rate at which traffic conditioning may be applied.
The per-port traffic conditioning features cannot be enabled on any of the interfaces when IPX routing
is enabled; IPX routing cannot be enabled when any of the per-port traffic conditioning features are
enabled on any of the interfaces. Transitioning between IPX routing and per-port traffic conditioning
involves dynamic downloading of new binaries to the network processor. We recommend that you
perform this dynamic downloading under light traffic conditions.
You can configure the Catalyst 4000 Layer 3 Services module to route using either
Caution
per-port traffic shaping or IPX, but not both. All standard QoS features support IPX
routing.
Because traffic shaping and policing configurations apply to the physical interface, you
Note
cannot configure per-port traffic conditioning features on subinterfaces.
Configuring Per-Port Input Rate Limiting
You can configure rate limiting on an input port on a per-physical port basis. The module monitors
traffic rate to verify conformity with the configured policing parameters. If the input traffic rate on a
port is nonconforming, the excess traffic is dropped. Input traffic that conforms to the policing
parameters is passed through the port without any changes. Input rate limiting applies to all the input
traffic and does not differentiate between various kinds of traffic, including traffic such as routing
updates.
Rate limiting is applied to all input traffic and is not confined to IP Layer 3 traffic. However,
high-priority traffic (such as routing updates or BPDUs) destined to the CPU is not subject to per-port
input rate limiting.
Installation and Configuration Note for the Catalyst 4000 Layer 3 Services Module
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