Emulating a WAN link on a LAN

Unanswered Question
Jun 11th, 2008

I would like to see if I can simulate a WAN link between a test LAN and our corporate LAN.

We are moving equipment to a datacentre and have been provisioned with a 20MB connection from our building to the datacentre.

I would like to set up a test switch in the same building as the equipment we want to move, and see if we can throttle the bandwidth between the two networks, to try and see what kind of slowdown we will get when moving from a LAN to WAN-based system.

We have Cisco Catalyst 3750G switches in both our LAN and WAN environments, so would hope to simulate the connection using these if possible.

I have this problem too.
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Joseph W. Doherty Wed, 06/11/2008 - 03:48

On a 3750G, force the interface to 100 Mbps, and then try "srr-queue bandwidth limit 15" on the interface. The limiter idles the port's bandwidth in actual incremements of about 6%, but 15 might provide about 21 Mbps. I.e. you may need to adjust the % up or down to come as close to your desired value as the hardware will support.)


BTW: there are PC free software tools that will emulate both a WAN's bandwidth, latency, and drops. You might want to consider using one of those.

Pravin Phadte Wed, 06/11/2008 - 04:35

I would agree with joseph.

This would be best possible way to simulate.

There are few more consideration that need to be looked forward. As above you can have a 100 Mbps link and squeeze it with rate-limit command.

I think when moving to the new link it would be a router? or will it be still on a switch ?

There would be a lot of difference when the WAN link would be on the router compared to on switch. The switching and routing would cause difference in latency, convergence for the link.

The type of WAN link will it be Fiber ? Fasteth ? BundleSerial ? makes a difference ?

You may have to get all these things also into consideration and come up with a average value.



hinareiCIEI Mon, 06/16/2008 - 08:28

Thank you joseph and pravin for your assistance.

After preliminary tests, joesph's suggestion seems to have returned satisfactory results. I am aware we will need to take into account latency between switches and routers when actually running live testing, but this is more than enough for a general starting point.

What has now arisen is the request for adding a 4510 into the equation at our building, connecting to a 3750 in the datacentre.

Could someone please let me know the best way to add a throttle to a port on a 4510, or to throttle inbound and outbound traffic on a port for a 3750G?

Pravin Phadte Mon, 06/16/2008 - 09:19


Thanks for keeping us informed.

The simple method i use is by rate-limit below is my example for 5 Mb.

Applied on the inetrface it works great.

rate-limit input 5242880 655000 655000 conform-action transmit exceed-action drop

rate-limit output 5242880 655000 655000 conform-action transmit exceed-action drop


5242880 bits

655360 Bytes


You also can you qos to limit the bandwidth.



hinareiCIEI Tue, 06/17/2008 - 04:12

thanks for your help so far!

I have looked at this method but cannot seem to find the command to change this on either the 3750 or the 4510

rate-limit doesn't seem to appear when configuring the interface. It's listed a show interface subcommand, but I cannot see how to configure this.

Do I have to set CAR or create policies first?

Pravin Phadte Tue, 06/17/2008 - 05:22

I am sorry but i took it as a router.

This command would depend on the ios verison on the switch.

This is what you can do for switch

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


interface FastEthernet 0/1


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.





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