Operating notes for rate-limiting

  • Rate-limiting operates on a per-port basis, regardless of traffic priority. Rate-limiting is available on all types of ports (other than trunked ports) and at all port speeds configurable for these switches.
  • Rate-limiting on a trunk is not allowed for the all, bcast, icmp, and mcast traffic types. Rate-limiting is not supported on ports configured in a trunk group (including mesh ports). Configuring a port for rate-limiting and then adding it to a trunk suspends rate-limiting on the port while it is in the trunk. Attempting to configure rate-limiting on a port that already belongs to a trunk generates the following message:<port-list> : Operation is not allowed for a trunked port.
  • Rate-limiting and hardware. The hardware will round the actual Kbps rate down to the nearest multiple of 64 Kbps.
  • Rate-limiting is visible as an outbound forwarding rate. Because inbound rate-limiting is performed on packets during packet-processing, it is not shown via the inbound drop counters. Instead, this limit is verifiable as the ratio of outbound traffic from an inbound rate-limited port versus the inbound rate. For outbound rate-limiting, the rate is visible as the percentage of available outbound bandwidth (assuming that the amount of requested traffic to be forwarded is larger than the rate-limit).
  • Operation with other features. Configuring rate-limiting on a port where other features affect port queue behavior (such as flow control) can result in the port not achieving its configured rate-limiting maximum. For example, in a situation where flow control is configured on a rate-limited port, there can be enough "back pressure" to hold high-priority inbound traffic from the upstream device or application to a rate that is lower than the configured rate limit. In this case, the inbound traffic flow does not reach the configured rate and lower priority traffic is not forwarded into the switch fabric from the rate-limited port. (This behavior is termed "head-of-line blocking" and is a well-known problem with flow-control.)In another type of situation, an outbound port can become oversubscribed by traffic received from multiple rate-limited ports. In this case, the actual rate for traffic on the rate-limited ports may be lower than configured because the total traffic load requested to the outbound port exceeds the port's bandwidth, and thus some requested traffic may be held off on inbound.
  • Traffic filters on rate-limited ports. Configuring a traffic filter on a port does not prevent the switch from including filtered traffic in the bandwidth-use measurement for rate-limiting when it is configured on the same port. For example, ACLs, source-port filters, protocol filters, and multicast filters are all included in bandwidth usage calculations.
  • Monitoring (mirroring) rate-limited interfaces. If monitoring is configured, packets dropped by rate-limiting on a monitored interface are still forwarded to the designated monitor port. (Monitoring shows what traffic is inbound on an interface, and is not affected by "drop" or "forward" decisions.)
  • Optimum rate-limiting operation. Optimum rate-limiting occurs with 64-byte packet sizes. Traffic with larger packet sizes can result in performance somewhat below the configured bandwidth. This is to ensure the strictest possible rate-limiting of all sizes of packets.

Rate-limiting is applied to the available bandwidth on a port and not to any specific applications running through the port. If the total bandwidth requested by all applications is less than the configured maximum rate, then no rate-limit can be applied. This situation occurs with a number of popular throughput-testing applications, as well as most regular network applications. Consider the following Example: that uses the minimum packet size:

The total available bandwidth on a 100 Mbps port "X" (allowing for Inter-packet Gap—IPG), with no rate-limiting restrictions, is:

(((100,000,000 bits) / 8 ) / 84) × 64 = 9,523,809 bytes per second


  • The divisor (84) includes the 12-byte IPG, 8-byte preamble, and 64-bytes of data required to transfer a 64-byte packet on a 100 Mbps link.

  • Calculated "bytes-per-second" includes packet headers and data. This value is the maximum "bytes-per-second" that 100 Mbps can support for minimum-sized packets.

Suppose port "X" is configured with a rate limit of 50% (4,761,904 bytes). If a throughput-testing application is the only application using the port and transmits 1 Mbyte of data through the port, it uses only 10.5% of the port's available bandwidth, and the rate-limit of 50% has no effect. This is because the maximum rate permitted (50%) exceeds the test application's bandwidth usage (126,642-164,062 bytes, depending upon packet size, which is only 1.3% to 1.7% of the available total). Before rate-limiting can occur, the test application's bandwidth usage must exceed 50% of the port's total available bandwidth. That is, to test the rate-limit setting, the following must be true:

bandwidth usage (0.50 × 9,523,809)