Management and Configuration Guide K/KA/KB.15.15

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 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 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 whereflow 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.
All traffic rate-limiting 185