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Port Traffic Controls
Rate-Limiting
0.5 Mbps of inbound traffic. If an interface experiences an inbound flow
of ICMP traffic in excess of its configured limit, the switch generates a log
message and an SNMP trap (if an SNMP trap receiver is configured).
■ ICMP rate-limiting is port-based: ICMP rate-limiting reflects the
available percentage of an interface’s entire inbound bandwidth. The rate
of inbound flow for traffic of a given priority and the rate of flow from an
ICMP rate-limited interface to a particular queue of an outbound interface
are not measures of the actual ICMP rate limit enforced on an interface.
■ Below-maximum rates: ICMP rate-limiting operates on a per-interface
basis, regardless of traffic priority. Configuring ICMP rate-limiting on an
interface where other features affect inbound port queue behavior (such
as flow control) can result in the interface not achieving its configured
ICMP rate-limiting maximum. For example, in some situations with flow
control configured on an ICMP rate-limited interface, there can be enough
“back pressure” to hold high-priority inbound traffic from the upstream
device or application to a rate that does not allow bandwidth for lower-
priority ICMP traffic. In this case, the inbound traffic flow may not permit
the forwarding of ICMP traffic into the switch fabric from the rate-limited
interface. (This behavior is termed “head-of-line blocking” and is a well-
known problem with flow-control.) In cases where both types of rate-
limiting (rate-limit all and rate-limit icmp) are configured on the same
interface, this situation is more likely to occur. In another type of situation,
an outbound interface can become oversubscribed by traffic received
from multiple ICMP rate-limited interfaces. In this case, the actual rate for
traffic on the rate-limited interfaces may be lower than configured
because the total traffic load requested to the outbound interface exceeds
the interface’s bandwidth, and thus some requested traffic may be held
off on inbound.
■ Monitoring (Mirroring) ICMP rate-limited interfaces: If monitoring
is configured, packets dropped by ICMP 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.)
■ 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 inbound bandwidth. This is
to ensure the strictest possible rate-limiting of all sizes of packets.
■ Outbound Traffic Flow: Configuring ICMP rate-limiting on an interface
does not control the rate of outbound traffic flow on the interface.
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