Port Traffic Controls

Rate-Limiting

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 for inbound and outbound traffic are separate features: The rate limits for each direction of traffic flow on the same port are configured separately—even the specified limits can be different.

■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 will still be forwarded to the designated monitor port. (Monitoring shows what traffic is inbound on an interface, and is not affected by “drop” or “forward” decisions.)

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