Enterasys Networks 2200 - Manual (Page 168)

Rate Limiting Configuration Screen
6-50 802.1 Configuration Menu Screens
In Multi-Dwelling-Units (MDU) or similar environments, the Rate Limiting feature can be
activated per port to adjust the usable bandwidth on a 10 Mbps Ethernet or other type of physical
connection. In residential housing, the service provider may offer multiple internet service
packages, each offering different bandwidth at a different price. These offerings can be supported
using low cost 10 Mbps Ethernet ports wired to each dwelling.
In the enterprise network, this feature (combined with Layer 3/4 prioritization) can provide
guaranteed delivery of high priority traffic through a congested network fabric. This is
accomplished through the construction of a committed information rate (CIR) fabric within the
traditional best effort enterprise LAN fabric.
Example
This is a simple example intended to show how the Rate Limiting feature can be applied to solve a
problem.
Assume that a network built using SmartSwitch 6000 chassis in the wiring closets are
interconnected with SmartSwitch Routers using Gigabit Ethernet links. Also, assume that 100
users are attached to each SmartSwitch 6000 chassis through 100 Mbps Ethernet ports. If each user
attempted to transfer data out of the wiring closet at the maximum possible rate, there could be up
to 10 Gbps (100 users x 100 Mbps) of traffic attempting to leave the chassis over a single gigabit
link. In this situation, much of the traffic will be arbitrarily dropped.
Now assume that the system administrator wants to guarantee the delivery of SAP R/3 traffic by
prioritizing it above all other incoming traffic to the chassis. (Unless the inbound rate of the SAP
traffic can be controlled, the guarantee still cannot be made because of the potential for
oversubscription of the inbound gigabit link by high priority traffic.)
To solve this problem, the Rate Limiting feature can be configured on each port to provide each
user with 5 Mbps of high priority bandwidth into the fabric. Now the maximum possible amount of
traffic attempting to leave the chassis at high priority is 5 x 100 = 500 Mbps. The gigabit link has
ample capacity to carry this load out of the chassis. Similar provisioning calculations must be
carried throughout the network all the way to the particular resources to which the service
guarantee applies. The sum of the rate limits on all user ports cannot exceed the capacity of the
weakest point in the delivery path to the resource.