Brocade BigIron RX Series Configuration Guide - Manual (Page 591)

BigIron RX Series Configuration Guide 513
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Configuration considerations
19
Credits and credit total
Each rate limiting policy is assigned a class. A class uses the average rate and maximum allowed
burst in the rate limiting policy to calculate credits and credit totals.
Credit size is measured in bytes. A credit is a forwarding allowance for a rate-limited port, and is the
smallest number of bytes that can be allowed during a rate limiting interval. The minimum credit
size can be 1 byte.
During a rate limiting interval, a port can send or receive only as many bytes as the port has Credits
for. For example, if an inbound rate limiting policy results in a port receiving two credits per rate
limiting interval, the port can send or receive a maximum of 2 bytes of data during that interval.
The credit size is calculated using the following algorithm.
Credit = (Average rate in bits per second)/(8*64453)
One second is divided into 64,453 intervals. In each interval, the number of bytes equal to the
credit size is added to the running total of the class. The running total of a class represents the
number of bytes that can be allowed to pass through without being subject to rate limiting.
The second parameter is the maximum credit total, which is also measured in bytes. The maximum
credit total is calculated using the following algorithm.
Maximum credit total = (Maximum burst in bits)/8
The running total can never exceed the maximum credit total. When packets arrive at the port, a
class is assigned to the packet, based on the rate limiting policies. If the running total of the class is
less than the size of the packet, then the packet is dropped. Otherwise, the size of the packet is
subtracted from the running total and the packet is forwarded. If there is no traffic that matches
rate limiting criteria, then the running total can increase until it reaches the maximum credit total.
Configuration considerations
• Except for port-based rate limiting policies, all rate limiting policy types can be applied only to
inbound ports on the device.
• Only one type of inbound rate limiting can be applied on a physical port. For example, you
cannot apply inbound port-and-ACL-based and inbound port-based rate limiting policies on the
same port.
• Outbound port-based rate limiting policy can be combined with any type of inbound rate
limiting policy.
• Any VLAN-based rate limiting can limit only tagged packets that match the VLAN ID specified in
the policy. Untagged packets are not subject to rate limiting.
• The minimum configurable rate limiting rate in the BigIron RX is 3000 bps. The maximum
configurable rate limiting rate is near line-rate.
• Control packets are not subject to rate limiting.
• Trunks with rate-limiting on their physical members is configurable where all the ports inherit
the rate-limiting policy.
• The device currently only support "strict" and "destination-weighted" scheduling schemes. (qos
scheduler ..) on the 16x10G card.