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Traffic passes through the rate limiter causing a draining of tokens. The drain rate
is dependent on how large the packets are and how much time elapses between
packets. At any given instant the level of tokens in each bucket is a function of the
fill rate, size of packets, and elapsed time between packets.
When packets are received on an interface with a rate limiter applied, the level of
tokens in each bucket dynamically changes in both of the following ways:
Hierarchical Rate Limits Overview
In another type of rate limiting, rate-limit hierarchies enable lower priority traffic to
access unused bandwidth allocated for real-time traffic, such as voice or video, during
times when no real-time traffic is flowing. IP subscribers receive multiple services,
such as Web, video, and file transfer, that have a maximum bandwidth. A rate-limit
hierarchy can apply a common rate limit to several classified flows, enabling them
to share bandwidth according to the preferences set in the hierarchical rate limits.
You can also use rate-limit hierarchies in a layer 2 (ATM) access network for DSL
where many routing gateways lead into one Broadband Access Server. The Broadband
Access Server uses rate-limit hierarchies to allocate shareable bandwidth to each
routing gateway, which enables unused bandwidth from one routing gateway to be
used by others. The hierarchy in the rate limit represents the hierarchy in the access
network.
Rate-limit hierarchies enable you to share unused bandwidth dynamically, taking
unused preferred bandwidth. They also enable real-time traffic to use all guaranteed
bandwidth at any time without violating the configured limit on the total interface
bandwidth. While preferred traffic fluctuates, the interface rate limit adjusts, dropping
non-preferred packets to keep the total flow through the interface under a configured
maximum rate, because preferred packets cannot be dropped by the shared rate
limits, only by their individual rate limits.
Shared rate limits in the hierarchy keep the combined traffic below a configured
maximum without dropping preferred packets. Preferred packets always reduce
tokens on these rate limits, making their token counts negative, if necessary. Later
non-preferred packets are then dropped in greater volume, bringing the total traffic
through the shared rate limit below its configured maximum.
Every packet passing through a rate limit hierarchy has an owner, which is the last
rate limit that can modify the packet; for example, by changing its color or dropping
it. Preferred packets are owned by their individual preferred rate limits, which do
not transfer ownership of the packet while the packet traverses the hierarchy.
Ownership of non-preferred packets is transferred while they move from one rate-limit
to the next in the hierarchy, so shared rate limits can change the packet color or
drop them.
Tokens are added every 100-ms sample period
Tokens are removed based on the size and rate of incoming packets
Chapter 5: Creating Rate-Limit Profiles
Hierarchical Rate Limits Overview
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