Figure 9: Comparing Burst Size Calculation Methods
Burst
Size
Bandwidth Limit
g041268
10 MTU
5 ms
For a policer burst-size limit calculated using the
5 ms
method, the effective burst-size
limit is proportional to the configured bandwidth limit. With a very low bandwidth limit,
the effective burst-size limit might be so small that the policer rate-limits traffic more
aggressively than desired. For example, a traffic “burst” consisting of two MTU-sized
packets might be rate-limited. In this scenario, a policer burst-size limit calculated using
the
10 MTU
method appears to be a better choice.
10 x MTU Method for Selecting Initial Burst Size for Gigabit Ethernet with 100 Kbps
Bandwidth
The following sequence illustrates the use of the 10 x MTU method for selecting an initial
burst size for test configurations for a Gigabit Ethernet interface configured with a
100 Kbps bandwidth limit:
1.
If you configure a 100 ms burst-size limit, the maximum amount of traffic allowed to
pass through the interface unrestricted is 1250 bytes, calculated as follows:
100,000 bps x 0.1 s
100 Kbps x 100 ms = ————————————————————— = 1250 bytes
8 bits per byte
2.
In theory, a 10 x MTU burst size would allow up to 15,000 bytes to pass unrestricted.
However, the maximum configurable burst-size limit for MX Series, M120, and M320
routers is 600 ms of the bandwidth limit. If you configure the maximum burst-size
limit of 600 ms of the bandwidth limit, the maximum amount of traffic allowed to
pass through the interface unrestricted is 7500 bytes, calculated as follows:
100,000 bps x 0.6 s
100 Kbps x 600 ms = ————————————————————— = 7500 bytes
8 bits per byte
On a Gigabit Ethernet interface, a configured burst-size limit of 600 ms creates a burst
duration of 60 µs at Gigabit Ethernet line rate, calculated as follows:
7500 bytes 60,000 bits
———————————— = ——————————————————— = 0.00006 s = 60 μs
1 Gbps 1,000,000,000 bps
33
Copyright © 2016, Juniper Networks, Inc.
Chapter 5: Implementing Traffic Policers on EX 9200 Switches
Summary of Contents for EX9200 Series
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