Re: VLAN questions - Page 3

dbarboza27 · ‎06-18-2010

Hi,

I have a couple of questions about VLAN.

1. Based in the following scenario:

Switch A ---Trunk---- Switch B ----Trunk------ Switch C

If I create a VLAN in swithes A and C (for reference VLAN 100), but I dont create it in Switch B, is it possible to communicate the hosts connected on Switch A in vlan 100 with the hosts connected to Vlan 100 in Switch B ?????

2. I know that the ethernet interfaces are broadcast. I supose that the VLAN interface are too, but I would like a second opinion about that.

Thanks for your comments,

Dasuntha Dinesh · ‎06-25-2010

Hi Jon,

As you know, by default Trunk Links are carrying all the VLAN traffics.

That's why we dont need to create Vlan 100 in switch B.

If you tested this scenario, you'll see it for sure.

Regards,

Dasuntha

Jon Marshall · ‎06-25-2010

Dasuntha_Dinesh wrote:

Hi Jon,

As you know, by default Trunk Links are carrying all the VLAN traffics.

That's why we dont need to create Vlan 100 in switch B.

If you tested this scenario, you'll see it for sure.

Regards,

Dasuntha

I'm not saying it doesn't work but i am looking for an explanation. Your trunk link from B to C is only forwarding vlan 1, you can see that from the output. There is no mention of that trunk link forwarding vlan 100 because you don't have a vlan 100 on switch B. So how does the frame get forwarded across the trunk to switch C in vlan 100 ?

To say a trunk forwards all vlans is correct but your output clearly shows the only active and forwarding vlan on switch B -> C trunk link is vlan 1.

Jon

Dasuntha Dinesh · ‎06-25-2010

Jon,

From below output showing the active vlans in the switch.

Port Vlans allowed and active in management domain

Fa0/5 1

Fa0/6 1

This output showing the allowed vlans in trunk.

Port        Vlans allowed on trunk
Fa0/5       1-1005
Fa0/6       1-1005

So, vlan 100 is allowed in the list & no need to configure vlan 100 in the switch B.

Dasuntha

Jon Marshall · ‎06-25-2010

Dasuntha

From your output -

Port        Vlans allowed and active in management domain
Fa0/5       1
Fa0/6       1

Port Vlans in spanning tree forwarding state and not pruned

Fa0/5 1

Fa0/6 1

B#

So on the trunk links between switch A -> B and B -> C vlan 100 is not forwarding on those trunk links. So if vlan 100 is not forwarding on either trunk link -

1) how does switch B know what to do with a frame tagged with vlan 100 from switch A

2) how does switch B know that it has to tag the frame going out to switch C with a vlan 100 tag when it isn't even aware of a vlan 100

Basically think of a L2 vlan between the 3 switches as a pipe per vlan. All a trunk link does is allow multiple pipes (ie vlans) on the same link. So a packet originated in vlan 100 is put into the vlan 100 pipe. It is sent to switch B but switch B has no pipe for vlan 100 so it cannot forward it. Compare this with vlan 1 where it is put into the vlan 1 pipe on switch A, switch B receives it and forwards it through it's vlan 1 pipe, and it is then sent to switch C via the trunk link which is simply part of the vlan 1 pipe.

But for vlan 100 you do not have a continuous pipe because switch B has no knowledge of vlan 100. So how is the packet being forwarded by switch B in vlan 100.

So it seems that switch B "blindly" receives and forwards all frames received on it's trunk link to all other trunk links regardless of the tag but this isn't the way switches are meant to work or at least not as far as i know. In other words i can tag the frame from switch A with any tag and it will forward it on the trunk link to switch C which is the logic of what you seem to be saying.

Am i understanding you correctly ?

Jon

Jon Marshall · ‎06-25-2010

Dasuntha_Dinesh wrote:

Jon,

From below output showing the active vlans in the switch.

Port Vlans allowed and active in management domain

Fa0/5 1

Fa0/6 1

This output showing the allowed vlans in trunk.

Port        Vlans allowed on trunk
Fa0/5       1-1005
Fa0/6       1-1005

So, vlan 100 is allowed in the list & no need to configure vlan 100 in the switch B.

Dasuntha

In addition to last post this shows quite clearly that only vlan 1 is active on switch B. So how does switch B know what to do with a packet received for vlan 100 when that is not an active vlan on switch B ?

Jon

Dasuntha Dinesh · ‎06-25-2010

Jon,

If you say that switches are not working like that, How can it work like this.

And switch B dont want to know the traffic passing by, because switch B dont have Vlan 100 active.

Switch B is passing all the traffic through the trunk ports.

And I searched alot & I was unable to find any where Cisco mentioned that, if you want to pass the vlan traffic from one switch to another switch through another switch, you need to create all the vlan in all the switches.

Because it's not practical in real environment. Every one knows that.

And some times remember that, the theory side can be different from the practical side.

I really dont want argue with you about this matter, because I tested it & I know that's working fine in Real Environment.

Anyway, nice to exchange the information like this.

Best Regards,

Dasuntha

Jon Marshall · ‎06-26-2010

Dasuntha

Because it's not practical in real environment. Every one knows that.

But everyone doesn't know that because that's what VTP is for ie. it propogates the vlans from a VTP server to the rest of the switches if they are VTP clients. So it's perfectly practical.

I really dont want argue with you about this matter, because I tested it & I know that's working fine in Real Environment.

My apologies if it seemed like i was arguing, that wasn't my intention. I find NetPro can be very useful in increasing my understanding of how things works and that is what i was trying to do. I enjoy talking things through with other experts and am perfectly happy if i am in the "wrong".

I too did some searching based on this thread both on Cisco site and google and have found references to both answers. I found one of Cisco press books that states clearly the vlan must be in the local switch database to be forwarded on that switches trunk links. I have found threads on study sites that say it must be created locally but have also found threads saying it doesn't. As you say theory is often different from practice.

The reason i am interested is that it has a direct effect on how traffic is forwarded across a switch.

Couple of things if you don't mind -

1) did you run this across real physical switches ? - if so which model/type

2) do you think it is worth me posting a question into LAN Switching and Routing to try and get a better understanding of how the switch is handling the traffic. I am still confused as to why your output on switch B shows vlan 1 is only active on the trunk links and yet vlan 100 is forwarded across the trunk link.

Once again, i appreciate being able to talk this through and am not concerned with being proved right or wrong, i simply want to understand how exactly this is working.

Jon

Dasuntha Dinesh · ‎06-26-2010

Hi Jon,

I didnt run this in real physical switches & planning to try it very soon.

I'll let you know if there's any update after that.

Of course you can post it in LAN Switching & Routing.

We can get more ideas from others & it'll help us to understand.

And I also agree with you that, always better to clarify & fully understand if we have any doubts in the mind.

For sure, I'll let u know if i have some information regarding this problem.

Have a Nice Weekend.

Regards,

Dasuntha