Operating notes for ICMP rate-limiting

ICMP rate-limiting operates on an interface (per-port) basis to allow, on average, the highest expected amount of legitimate, inbound ICMP traffic.

  • Interface support: ICMP rate-limiting is available on all types of ports (other than trunk ports or mesh ports), and at all port speeds configurable for the switch.

  • Rate-limiting is not permitted on mesh ports: Either type of rate-limiting (all-traffic or ICMP) can reduce the efficiency of paths through a mesh domain.

  • ICMP percentage-based rate-limits are calculated as a percentage of the negotiated link speed: For example, if a 100 Mbps port negotiates a link to another switch at 100 Mbps and is ICMP rate-limit configured at 5%, the inbound ICMP traffic flow through that port is limited to 5 Mbps. Similarly, if the same port negotiates a 10 Mbps link, it allows 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 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 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.