Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring ACLs (Rev. 03)
Overview
© 2008 Foundry Networks, Inc
Page 7 of 50
Traffic Rate Limit
Traffic congestion is a problem that affects many different types of networks. The problem usually
arises on network uplink ports that serve as aggregation points for all other nodes in the network.
Although uplink ports have memory buffers for outbound and inbound traffic, heavy network
traffic loads can fill the buffers (usually the outbound memory buffer). When the buffers are full
there is a short pause, during which outbound traffic is transmitted and no new outbound traffic is
accepted. This allows the port to make room in the buffer for additional packets.
On most enterprise networks congestion is rarely a problem, because most end users do not use
time sensitive applications. Inherently, time-sensitive applications make up a very small amount of
enterprise network bandwidth, so they can easily be accommodated.
The converse is true in a service provider network, where there is a large mass of end users
involved in various time-sensitive applications such as Internet voice calls and online gaming.
Because most users on service provider networks only have limited bandwidth, congestion
problems occur more frequently than in enterprise networks.
Ethernet technology was developed for shared networks, which means that no specific limits were
put on the bandwidth capacity allocated to a single user at any given moment. Also, Ethernet
granularity of 10, 100, or 1000 Mbps line speeds is not optimized for the bandwidth aggregation
requirements of service provider networks. Additional functionality that was capable of using
partial Ethernet line speeds was required.
Rate Limiting was created exactly for this purpose - to help control congestion on service provider
networks, and to help ensure proper use of bandwidth resources. Rate Limiting and Traffic Shaping
allow network administrators to allocate specific amounts of bandwidth per user or flow. Using
rate limiting can ensure that there is no needless congestion in the network, and that bandwidth is
aggregated properly toward the core.
Rate limiting is configured by specifying a data rate when applying an ACL to the flow. Using
ACL conditions, different rates can be assigned for different flows.
A common use of rate limiting or traffic shaping is to provide Internet services to complexes of
apartment buildings.
Figure 2
shows an example of this kind of service. Any number of apartments
can be connected with different rates although they are connected to the same device. In building 1
for example, two apartments are connected to the device, one at an internet speed of 64Kbps and
the other at an Internet speed of 256Kbps.