Hi churchillma,
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
