Port-Rate Limiting, Policing, and Shaping

The switch QoS implementation supports the following two features for bandwidth management and traffic control:

• ingress port–rate limiting—a mechanism to limit the traffic rate accepted by the specified ingress port

• egress port-rate shaping—the process by which the system delays and transmits packets to produce an even and predictable flow rate

  Each port has eight unicast and multicast queues, Class of Service (CoS) 0 to CoS 7. Traffic shaping exists on the egress CoS 6 and CoS 7, but you cannot change the configuration. CoS 6 and CoS 7 are strict priority queues, with traffic shaping for CoS 6 at 50 percent and CoS 7 to five percent of line rate.

  Some VOSS hardware platforms allow you to configure an egress shaping rate for each port manually. For XA1400 Series, the egress shaping rate for each front panel port dynamically adjusts to the auto-negotiated link speed, up to the maximum link speed of the port.

The VSP 4450 Series switch QoS implementation supports the following two features for bandwidth management and traffic control:

• ingress traffic policing—a mechanism to limit the number of packets in a stream that matches a particular classification

• egress traffic shaping—the process by which the system delays (or drops) and transmits packets to produce an even and predictable flow rate

Each feature is important to deliver DiffServ within a QoS network domain.

Token buckets

Tokens are a key concept in traffic control. A port-rate limiter, policer, or shaper calculates the number of packets that passed, and at what data rate. Each packet corresponds to a token, and the port-rate limiter, policer, or shaper transmits or passes the packet if the token is available. For more information, see Token flow.

The token container is like a bucket. In this view, the bucket represents both the number of tokens that a port-rate limiter, policer, or shaper can use instantaneously (the depth of the bucket) and the rate at which the tokens replenish (how fast the bucket refills).

Each policer has two token buckets: one for the peak rate and the other for the service rate. The following figure shows the flow of tokens.

Token flow