HP 2910AL - Manual (Page 317)

Note on Testing
Rate-Limiting
Port Traffic Controls
Rate-Limiting
Rate-limiting is applied to the available bandwidth on a port, and not to any
specific applications running through the port. If the total bandwidth
requested by all applications is less than the configured maximum rate, then
no rate-limit can be applied. This situation occurs with a number of popular
throughput-testing applications, as well as most regular network applications.
Consider the following example that uses the minimum packet size:
The total available bandwidth on a 100 Mbps port “X” (allowing for Inter-
packet Gap—IPG), with no rate-limiting restrictions, is:
(((100,000,000 bits) / 8 ) / 84) x 64 = 9,523,809 bytes per second
where:
– The divisor (84) includes the 12-byte IPG, 8-byte preamble, and 64­
bytes of data required to transfer a 64-byte packet on a 100 Mbps link.
– Calculated “bytes-per-second” includes packet headers and data. This
value is the maximum “bytes-per-second” that 100 Mbps can support
for minimum-sized packets.
Suppose port “X” is configured with a rate limit of 50% (4,761,904 bytes).
If a throughput-testing application is the only application using the port,
and transmits 1 Mbyte of data through the port, it uses only 10.5% of the
port’s available bandwidth, and the rate-limit of 50% has no effect. This
is because the maximum rate permitted (50%) exceeds the test applica­
tion’s bandwidth usage (126,642-164,062 bytes, depending upon packet
size, which is only 1.3-1.7% of the available total). Before rate-limiting can
occur, the test application’s bandwidth usage must exceed 50% of the
port’s total available bandwidth. That is, to test the rate-limit setting, the
following must be true:
bandwidth usage > (0.50 x 9,523,809)
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