Category: Cisco

Limit traffic on a Cisco switch L2 port with minimal configuration

Let’s say that somebody (or some task in a test) ask you to limit the inbound traffic on a switch Layer 2 port by using minimal configuration possible. I must say that in the first steps I failed this task miserable, but actually is very simple to do it.

I will use a plain layer 2 Cisco 2950 switch for this task. I observed that I could not implement this on a Cisco 3500XL. I don’t know if the IOS image was wrong, but I didn’t investigate too much in that area as I cannot stand 3500XL switches and they are actually pretty old piece of hardware.

No topology is needed for this as I will only show how to do it and not testing it with real traffic. I will do testing later when I’ll have some more time, or you can do it on your own.

See the tutorial below:

Cisco hidden tool: test crash

This is pretty old trick, so maybe  you already know it, but for the rest of you, it can be interresting.

You just had a crash on you Cisco hardware and you have the logs, but don’t know exactly what caused this crash. You tried to have the device crash again, to compare the logs maybe you’ll find the cause, but your device won’t crash (of course, it will crash only when don’t want that to happen).

There is a hidden Cisco command: “test crash”. This can help you if you are lucky enough to have the real crash exactly like one of those you can test with “test crash” command.

Note: As you can see in the previous posts I do my tutorials based mostly on Dynamips, when it is possible. This is not the case. So, please do not use Dynamips for this test, as it will go into errors since the device is simulated and not a real one.

Please see the tutorial below:

Frame-Relay: PVC test with ping to own interface

Let’s say that you have a link configured with Frame-Relay and from time to time you observe that the link is having some strange problems. Since you cannot be sure that the link is provided 100% error free you want to do your own tests. You want to see if the Frame-Relay between R0 and R1 is correctly configured. In order to verify this you want to set up the R0 to test the PVC to R1, by sending traffic to its own IP address.

Please download the topology here. The Frame-Relay between R0 and R1 is already configured.

See the tutorial below:

Cisco hidden tools: Testing TCP connection

Sometimes you have a connection between 2 routers that you want to test with something more than a ping. Cisco devices have a hidden command which is actually a very powerful testing tool for your TCP connection.

The command is “ttcp” and you will not find it in the the default list of commands of a Cisco device. So, even if you use help ( “?” mark at the router prompt) this command will not be showed to you.

For our testing we will use the same topology as in the previous posts. If you do not have it please download it here. Since this is a point-to-point TCP connection testing we will not use any fancy routing protocol or other networking protocols.

See the tutorial below:

How to force Loopack interface to be advertised with a /24 netmask

Many of you already know that usually when you advertise a Loopback interface into OSPF it is advertised with /32 netmask (one IP address) even if the IP address under Loopback interface is with a /24 netmask. If you checked my previous post, you’ll see at the end that I’m right, and the Lo0 address is advertised with /32 netmask.

But what if you have to advertised as a /24 subnet (task require or some testing) ? There is “dirty little trick” to do it. I will use the same topology and design like in the post before. If you do no have the topology please download it here.

Watch the movie: