RTS DV-3400 Technical Information PDF herunterladen (Seite 336)

Port Traffic Controls

All-Traffic Rate-Limiting for the 5300xl, 3400cl and 6400cl Switches

■ Interface Support: ICMP rate-limiting is available on all types of ports

and trunks on the switches covered by this guide, and at all port speeds

configurable for these devices.

■ Rate-Limiting Not Permitted on Mesh Ports: Either type of rate-

limiting can reduce the efficiency of paths through a mesh domain.

■ Monitoring (Mirroring) ICMP Rate-Limited Interfaces: If monitor-

ing is configured, packets dropped by ICMP rate-limiting on a monitored

interface will still be forwarded to the designated monitor port. (Monitor-

ing shows what traffic is inbound on an interface, and is not affected by

“drop” or “forward” decisions.)

■ ICMP Rate-Limit Imposes an Average Bandwidth Limit: The config-

ured ICMP rate limit on an interface reflects the permitted average

forwarding rate for ICMP traffic from the interface to the switching fabric.

(Note that while occasional bursts of traffic above the configured rate may

be observed, the average rate will conform to the configured limit). Rate-

Limiting is packet-based, and is calculated internally as the maximum

number of 64-byte packets that can be forwarded within the configured

bandwidth percentage. Where traffic includes packets larger than 64

bytes, actual average rates may be lower than the configured rate. Also,

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.

■ Network Stress Conditions: Under normal network operating condi-

tions, ICMP rate-limiting limits inbound traffic on an interface to no more

than the configured level. However, under network stress conditions, the

interface may allow occasional, brief bursts of inbound traffic forwarding

that exceed the configured rate.

■ 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 situa-

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