How Fixed Rate Limiting Works - Foundry Networks Switch and Router Installation And Configuration Manual

Foundry Switch and Router Installation and Configuration Guide
Table 14.1: Rate Limiting Support on Foundry Products (Continued)
Product Software
TurboIron/8 BIP (IP-only Layer 3
Layer 3 Switch)
Switch
TurboIron/8 BIS (Layer 2 Switch)
Layer 2
Switch
Stackable NIP (IP-only Layer 3
NetIron Layer Switch)
3 Switch
Stackable FWS (Layer 2
FastIron Switch)
Workgroup
Switch
a.You can apply a rate limit to all the interface's IP traffic or you can refine the rate limiting by
applying a rate-limiting ACL to the interface.
Fixed Rate Limiting
Fixed Rate Limiting allows you to specify the maximum number of bytes a given port can send or receive. The
port drops bytes that exceed the limit you specify. You can configure a Fixed Rate Limiting policy on a port's
inbound or outbound direction. The rate limit applies only to the direction you specify.
Fixed Rate Limiting applies to all types of traffic on the port.
When you specify the maximum number of bytes, you specify it in bits per second (bps). The Fixed Rate Limiting
policy applies to one-second intervals and allows the port to send or receive the number of bytes you specify in
the policy, but drops additional bytes.
NOTE: Foundry recommends that you do not use Fixed Rate Limiting on ports that send or receive route control
traffic or Spanning Tree Protocol (STP) control traffic. If the port drops control packets due to the Fixed Rate
Limiting policy, routing or STP can be disrupted.

How Fixed Rate Limiting Works

Fixed Rate Limiting counts the number of bytes that a port either sends or receives, in one second intervals. The
direction that the software monitors depends on the direction you specify when you configure the rate limit on the
port. If the number of bytes exceeds the maximum number you specify when you configure the rate, the port
drops all further packets for the rate-limited direction, for the duration of the one-second interval.
Once the one-second interval is complete, the port clears the counter and re-enables traffic.
Figure 14.1 shows an example of how Fixed Rate Limiting works. In this example, a Fixed Rate Limiting policy is
applied to a port to limit the inbound traffic to 500000 bits (62500 bytes) a second. During the first two one-second
intervals, the port receives less than 500000 bits in each interval. However, the port receives more than 500000
bits during the third and fourth one-second intervals, and consequently drops the excess traffic.
14 - 2
Fixed Adaptive Rate Limiting
Rate
Limiting
Layer 3
a
port
X
X
X
Layer 3 Layer 2
Virtual VLAN
a
Interface
X X
X
Layer 2
a
port
X
December 2000