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.
156
Aruba 2930F / 2930M Management and Configuration Guide
for ArubaOS-Switch 16.08
