Solved: Unknown vlan in trunk

Olga.Vedina · ‎09-02-2005

Hi all!

We have mostly 2950 switches with standard image and 2950lre with EI. All switches in transparent modes with different domain names.

Here is the problem

cat1 <-trunk-> cat2 <-trunk> cat3

On cat1 and cat3 there are ports in ,let's say ,vlan 10

Right now I have to create vlan 10 in transit cat2. Otherwise it won't pass tagged packets recieved from cat1 to cat3.

I was under impression that catalysts can pass all vlans in trunk by default,even if it's not known on local switch.

I did debug on lre (debug switch vlan) and after creating vlan, it put trunk port in tagged mode for this vlan

VLANDEBUG:STP_FORWARDING: vlan 1289 port 25

strata_add_port_to_vlan: adding tagged port for VM_1Q_PORT

VLANDEBUG:Set Tagged Mode For Port:25, Unit:0

So, after creating vlan it permits this vlan on trunk port

I wonder if it's platrofm depended, IOS image (c2950lre-i6l2q4-mz.121-22.EA1) depended or it's just the way catalysts work?

amit-singh · ‎09-02-2005

Hi,

You have to create Vlan 10 on your Cat2 otherwise it will not pass the traffic for the Vlan2. Think it in this way that if your switch is a VTP client and you have a VTP server, now your VLAN info will be passed to every VTP client and thus you have the same number of VLANS on the switch and hence the traffic gets pass over the trunk. If you have switches in Transparent mode than each switch has its own Vlan database and to get the traffic passed fo a particular Vlan that vlan should exist in Vlan database of each and every switch,.

If that vlan doesnot exist then how the switch will tag the packets when it has to send the traffic to other switch. On catalyst switch the moment the tagged packet exists the trunk port it rips off the dot1q TAG and adds a Port vlan ID i.e vlan 10 on the packet and the moment it enters on the trunk port to pass it to other link, a DOT 1q tag is added on the switch. This is how the catalyst switch works. So if doesnot have any Vlan in its vlan database it will not pass the traffic for that Vlan.

I remeber I did the same LAB when I was giving a training to some Army guys and it happend to be the same.

regards,

-amit singh

View solution in original post

Olga.Vedina · ‎09-02-2005

Answering myself :)

I guess it has to do with STP. As stp mode by default is PVSTP, switch needs to know vlan id to build spanning tree for this vlan. Before that it will drop the packet as it can be looped.

So, I think it's the right behavour for switch operating in pvst mode

amit-singh · ‎09-02-2005

Hi,

You have to create Vlan 10 on your Cat2 otherwise it will not pass the traffic for the Vlan2. Think it in this way that if your switch is a VTP client and you have a VTP server, now your VLAN info will be passed to every VTP client and thus you have the same number of VLANS on the switch and hence the traffic gets pass over the trunk. If you have switches in Transparent mode than each switch has its own Vlan database and to get the traffic passed fo a particular Vlan that vlan should exist in Vlan database of each and every switch,.

If that vlan doesnot exist then how the switch will tag the packets when it has to send the traffic to other switch. On catalyst switch the moment the tagged packet exists the trunk port it rips off the dot1q TAG and adds a Port vlan ID i.e vlan 10 on the packet and the moment it enters on the trunk port to pass it to other link, a DOT 1q tag is added on the switch. This is how the catalyst switch works. So if doesnot have any Vlan in its vlan database it will not pass the traffic for that Vlan.

I remeber I did the same LAB when I was giving a training to some Army guys and it happend to be the same.

regards,

-amit singh

Olga.Vedina · ‎09-02-2005

Oh,thank you! It clears it up!