Limit bandwidth on a Cisco Catalyst switch port

Why should you limit bandwidth?

There are a number of reasons for limiting bandwidth on a Cisco switch Ethernet port. (Of course, the port speed of the switch port always poses an ultimate limitation.)

For example, let's say you work for an Internet service provider in a colocation facility, and you sell access to the Internet in 1-Mb increments. The switch port in question is 100-Mb full duplex. In this case, you would limit the end device connected to that switch port to whatever 1-Mb increment the customer has paid for.

This is just one example of how this configuration could be useful, but there are many other possibilities. Let's look at the commands used to create this bandwidth limit on a Cisco switch.

How do I configure a bandwidth limit?

In the Cisco IOS on a Catalyst switch (not on a router), there's an Interface Mode command called shape round-robin queue bandwidth. More specifically, the command is srr-queue bandwidth.

This command has been around since IOS 12.2(25). For these examples, I'm using a Cisco Catalyst 2960 switch.

Entering the command appended with a question mark will display the command options. Here's an example:

Switch(config)# interface FastEthernet 0/1

Switch(config-if)# srr-queue bandwidth ?

limit: Configure bandwidth-limit for this interface

shape: Configure shaping on transmit queues

share: Configure shared bandwidth

As you can see, the command options are limit, shape, and share. While we'll focus on the limit option this time, keep in mind that you can also use the srr-queue bandwidth command to shape and share bandwidth. For example, let's say you have a 100-Mb Ethernet port on a Catalyst switch. You're selling the bandwidth on the port, and a customer has bought 10 Mb of bandwidth. Obviously, you want to limit the outbound bandwidth on the port to 10 Mb instead of the full 100 Mb.

To do so, go to Interface Configuration Mode on the switch port, and apply the srr-queue bandwidth limit command. Here's an example:

Switch(config)#interface FastEthernet 0/1

Switch(config-if)# srr-queue bandwidth limit 90

The 90 sets the outbound bandwidth limit on the port to 90 percent of the port speed. Since this is a 100-Mb port, this should limit the outbound traffic from the port to 10 Mb.

Variations on bandwidth limiting

As with just about everything in the Cisco IOS, there are multiple ways to accomplish the same thing. One way would be to manipulate the port speed. But if you want to hard-code the port speed on the Ethernet port to 10 Mb, you could also limit the customer to only 10 Mb of bandwidth using the speed 10 command. However, you probably won't get the same level of performance from the 10-Mb Ethernet port. What if you want a port speed that's less than 10 Mb? You could limit the port speed to 10 Mb and then use the srr-queue bandwidth limit 90 command to limit the outbound speed of the port to only 1 Mb.

What if you're working with a Cisco router instead of a switch? Cisco routers don't support the srr-queue command. While there are several different QoS options that might accomplish the same thing, a common method that I've used is the rate-limit command.

For example, on a Cisco router when in Interface Mode, you could limit outbound bandwidth to 1 Mb using the following command:

Router(config-if)# rate-limit output 10000000 2000 2000 conform-action continue exceed-action drop

Summary

Whether it's inbound or outbound, being able to control bandwidth on a Cisco switch or router port is a very valuable skill. No matter how difficult or simple the task, knowing the right command is usually the key to getting the task completed when working with the Cisco IOS. For more information on using the srr-queue bandwidth command, see Cisco's command reference or configuration guide.