I'm playing with a Catalyst 3500 and a Cisco 877 tring to understand how the VTP pruning works. I would like to see which tagged packets pass through the trunk link but I don't find out any useful command under the debug menu.
my first goal is to understand how pruning works. It is the next step after configuring VLANs manually. For that I can use virtual interfaces and see whether the ping pass through or not. But the pruning is quite a bit different, because as soon as an interface (either physical or virtual) is associated with a VLAN the switch enable the trunk to allow tags associated with that VLAN to pass through the link. So ping is not quite useful and I though to go just a layer below. I though I could enable a debug that told me "hey a
802.1q frame has arrived with tag number #" and so on. As far as I can see there is no way to test the pruning method and I must rely on which the show commands say to me, don't I?
Furthermore I thought I could use a device that broacasts tagged frames on all the interfaces it has (just as it is for the hub that repeats all frames on all ports but the source) but I think such a device doesn't exist, does it? It could exists just for test purposes...
As you can guess I would like to see by my eyes that what the book says is the truth ;-)
unfortunately that command is not supported on our 4006. I believe Cisco COS provides all the tools needed to understand how their devices are working. But, let me say that, sometimes you must already know both the theory and what the output of sh means in order to understand what's going on. And when you study a topic for the first time you don't know which commands could help you and so my way of studying the system is to treat it like it was a "black box" and in some way not trusting it.
I saw that a switch doesn't forward 802.1Q frames (I thought the opposite but I was wrong). Can perhaps a hub help me? Sorry I still want to proceed my way and to learn by my errors ;-). I'm going to use 3 straight-forward cables, 2 that connect the trunking ports to the hub and the third from the hub to a PC with a 802.1Q sniffer installed on it.
This approach will result in you becoming expert in the field.
I was wrong). Can perhaps a hub help me?
to the hub and the third from the hub to a
My guess is that a hub in not likely to help.
ask on comp.dcom.lans.ethernet.
This is because 802.1q frames are 4 bytes longer that the standard alllows and I guess that a hub will (OK may) not pass them through.
If you have a hub, worth a try though.
Cisco switches can I believe be thought of as dealing with 802.1q frames as follows.
On entry to the switch the 802.1q header is stripped from the frame however the information contained within the header in some proprietary way remains
associated with the frame within the switch.
Various processing is carried out on the frame bearing in mind the
The frame then may be forwarded. In this case the 802.1q information is then used to decide whether to create a new 802.1q header. For example if the exit port is an "access" port then no header is needed and a similar case exists if the frame is in the native vlan of an exit trunk port.
Hope that helps.
There is now a lot of good material on the cisco web site.