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found problem to carry certain vlans a cross 2 Switches

I have core Switch 6506 series at HO location, OSPF applied on the Switch to advertise all networks to remote routers locations which ospf applied as well on all routers that has VSIs.

Now, in between, I mean between HO switch to all routers at remote locations, I have 2950 switch, which carries the vlan traffic from/to each others.

I have assigned the port f0/23 on 2950 switch which connected to port 2/23 6506 switch as trunk port as well as 6500 port 2/23 and allows the configured vlans.

the remote routers has (SVI) and configured as:

(conf)int vlan x

ip address x.x.x.x x.x.x.x

(conf)int vlan y

ip address y.y.y.y y.y.y.y

(conf) router ospf (process id)

network x.x.x.x (etc)

network y.y.y.y (etc)

the same likely Scenario apllied on 6500 series.

Now, my concern about the 2950 Switch in between, the trunk between 2950 to 6500 is configured to allow native vlan x, it works fine, but if I remove the Native Vlan & configure the port to allow the same vlan as tagged vlan i.e: switchport trunk allow vlan x, then the traffic is discarded, As I noticed i can't pass more than one native vlan through a trunk, and we might need multiple vlans to be carried out for multiple links.

Now, my question is How would I let specific Vlans tagged to be be passed from 6500 till it reaches their remote locations without usin the Native Vlan (Number) config on both Switches.

Appreciate ur feedback & help, since this problem puzzled me a lot.


New Member

Re: found problem to carry certain vlans a cross 2 Switches

For trunking between Cat 2950 and 6500 and passing more than ove lan traffic , check the below link which has the

New Member

Re: found problem to carry certain vlans a cross 2 Switches


By default, an 802.1Q trunk passes all VLANs, if you have not already configured "switchport trunk allowed vlan...". If you already configured "switchport trunk allowed vlan...", make sure you add to the list, at both ends, those VLANs that you want to carry across the trunk.

By default, the Native VLAN on an 802.1Q trunk is VLAN 1. If you want to change it to another VLAN, just make sure you configure the same Native VLAN at both ends of the trunk using the command "switchport trunk native vlan ".

You may or may not choose to configure your VLAN x or y as the Native VLAN. Whichever VLAN it is, the rule of thumb is make sure it's the same Native VLAN at both ends of the trunk.


Thank you.


Lim TS

New Member

Re: found problem to carry certain vlans a cross 2 Switches

I think I may be missing something here but a trunk is only between the two devices, the 2950 and the 6509. so you set the port on the 2950 to be a trunk and the port on the 6509 as a trunk (use the same vlan as the native vlan preferably not even a vlan that is going to carry traffic) and then allow all the vlans that need to go thru that trunk.

the port on the 2950 and the 6509 have to be switch ports that are set up for trunking neither one can be a layer 3 interface.

Re: found problem to carry certain vlans a cross 2 Switches

Hi Dabels,

let me know what are u missing exactly, so that I can explain it clearly enough....

Actually, I didn't get your point, but i could imagine that u meant to say the ports shouldn't be configured as layer 3 interfaces.

Here is the Scenario:-

Offcourse the 6509 is performing as a router & doing inter-vlan-routing in order for all networks reching each other.

I have certain Networks on 6509, as well as having certain Networks in a remote side router, I need both Networks to be able accessing each other.

Now, the media type in between (link) 6509 and other remote router is basicalley layer 2 setup.

showing it briefly:

L3(router)-------L2(2950 Switch)---------L3(core Switch 6509).

The router has ethernet switching module (SVI), already there is another link between the router to the core Switch provided by (Wireless P-to-P link).

Our link is going to be the 2nd link, remote site router & core switch are running OSPF protocol to advertise both Networks besides for redundncy issue, if one of the links goes down the other is up.

Now, all my concern about the ability of carrying 2nd link traffic between HO to Remote location.

Here we are providing layer 2 carrier only for all customers,in this case we basically carrying traffic from HQ to its remote site through purely layer 2 setup, Simply we usually use to assign specific vlan in customer side as well as in our layer 2 switches passing through our wireless bridges to the customer end(by vlan tagging), our wireless bridges supports vlans as well.

Now, we have created vlan 703 for this client in his core switch, we then configured port 2/23 as trunk port using dot1q encapsulation & only allows vlan 703. then a cable is connected from 2950 switch port f0/23 which configured as well in trunking mode and allowing only vlan 703, the traffic goes through our wireless devices which passes vlan 703 till reach the othe remote router.


in order for 6506 to communicte with remote site router, we have created layer 3 vlan interfaces on both and give them IP's within the same Subnet which then after both Networks on both sides are advertised and accessable for each other.


Now, If I want to activate another link for the same enterprise, I have to assign another vlan ID, but the problem now is vlan 703 is configured native already in 2950, if I turned it to non native then the link is down.even though, what should I do if I need to pass another vlan through the same trunk??

How would I pass vlan 703 without native command on 2950??? Am I doing some thing wrong??

Hope Now u got my point, pls let me know ur feedback,


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