Difference between specific Vlan Config

[Mohamed Sobair]

Hi All,

I would like to know the following:-

1- when I create subinterface on router with encabsulation dot1q vlan num, as well as execute the commands bellow:- for example:-

a) (conf)int f0/1.1

enacapsulation dot1q 100

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

and executing the following:

b) (conf)interface vlan 100

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

what is the difference between the both above? and is vlan 100 on example (a)tagged & (b) is not tagged?

Any ideas about the difference.

thank you,

[pkhatri]

The first config (a) is used on a routed port in a router, which is configured to carry 802.1q-tagged traffic. This config simply associates all tagged traffic with a 802.1q VLAN ID of 100 with that sub-interface. The traffic is tagged since this is a trunk port..

Your config in (b) can be used on a switch or a router with an ethernet switching module. Such an interface is called a Switched Virtual Interface (SVI) and it provudes layer-3 termination on the router/switch for that VLAN. The VLAN number that it is associated with is the number that follows 'interface vlan'. On a switch/router, you would do this to allow inter-vlan routing. This is a software interface not a physical one. By definition, the traffic that this interface terminates is tagged.

Pls do remember to rate posts.

Paresh

[Mohamed Sobair]

Hi Paresh,

on one I agree with u, cause I've done it my self, but on (b), I noticed when I've done it on 4500 series switch associated with truk port to other switch, but it carries native traffic when the following applied:-

(conf)int vlan 100

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

Do u know why? as u said by diffinition this interface is tagged.

thanks

[pkhatri]

Ok, there's an exception. If the VLAN associated with that interface is also the native VLAN, then the traffic will be untagged.

Paresh

[Mohamed Sobair]

Hi Paresh,

ok, Now I am confused and lets sort this out, I will give u an example and u just show me the solution:-

I have core Switch 6500 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.

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 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 set only with native vlan, and if I set without Native Vlan then the traffic is discarded. How would I let specific Vlan tagged from 6500 till it reaches their remote locations without usin the Native Vlan (Number) config on both Switches.

Thanks.

[pkhatri]

I'm not quite sure what you mean when you say "the trunk between 2950 to 6500 is set only with native vlan".

Could you clarify that for me ?

Paresh

[Mohamed Sobair]

Hi Paresh,

ok, the Setup as follows:-

I need to carry certain vlans lets say 2 seperat vlans (100/101) from the Core Switch port through the trunk port assigned on 2950 switch.

Now, If I execute the command bellow on the 2950 switch port which is trunk already as follows:-

(conf)int f0/23

switch port mode trunk

switch port trunk allowed vlan 100,101

then the traffic is discarded, but if I set the same port as follows:-

switchport trunk native vlan 100

switchport trunk native vlan 101

then the link is up.

*The Vlan 1 on 2950 is shutdown

the configuration on 6500 port which connected to the 2950 trunk port as follows:-

enable> set trunk 2/23 nonegotiate dot1q

enable> set vlan 100

enable> set vlan 110

already Ip addresses also assigned under the above Vlan Interfaces.

Now, 2950 trunk port is only passing vlan traffic to/from end routers to the HO Switch.

Is there any way to pass tagged traffic from 6500 switch through 2950 switch port without the Native Vlan command? and is there any thing neede to be done on 6500 switch?

let me know if u have some doughts.

thanks.

[mahmoodmkl]

HI

As u said that u have shutdown the SVI of vlan 1 then what is the management vlan interface on 2950.because by default the native vlan of the 2950 will be vlan 1.and what is the native vlan of 2/33 on 6500.can u configure like this and try

on 6500

set trunk 2/33 on dot1q nativevlan,100-101

clear trunk 2/33 then the vlans

on 2950

switchport mode trunk

switchport trunk encapsulation dot1q

switchport trunk allowed vlan nativevlan,100,101

swithcport trunk native vlan vlanname

hope this helps

Mahmood

[amit-singh]

Hi,

What's Happening in this case, you have configured trunk on 6500 and also the respective vlans. The trunk port is 2/23 but still you have the native vlan as 1 on 6500.

On the other hand when you are setting up the 2950, you are giving the native vlan as 100 and 101 which is being different on the trunk link between 6500 and 2950.

As there can be only 1 native vlan on the trunk what I would suggest you to do the following:

On Cat 6500 Do the following:

enable> set trunk 2/23 nonegotiate dot1q

enable> set vlan 100

enable> set vlan 110

enable> set vlan 100 2/23 ----> This command will make vlan 100 as the native vlan on the trunk on 6500.

now on 2950 do the following:

conf)int f0/23

switch port mode trunk

switch port trunk allowed vlan 100,101

switchport trunk native vlan 100

This will work for sure. Let me know if you face any problem.

HTH, Please rate if it does.

-amit singh

[Mohamed Sobair]

Hi amit,

ok, but if we have in future other vlans to be configured, as u have said the trunk can't carry more than one vlan how would the setup be?

I mean if we are now decided to add vlans (110,120) to be carried out through the trunk, what would be the Setup?

thanks,

[amit-singh]

Hi,

I didnot say that you caanot carry more than one vlan on the trunk.

You cannot have more than one Native vlan on the trunk but you can carry multiple number of vlan on the trunk. Even the max, that switch supports.

There is a diffrence between the native vlan and a normal vlan. The native vlan frames as sent untagged on the trunk while other vlans are tagged.

HTH, please rate if it does.

-amit singh

[Mohamed Sobair]

Hi Amit,

Sorry, it was my mistyping mistake, I meant to say, if we decided to carry out more than one native vlan, as if we decided to configure vlan (110,120) for example as to applicable and applied for the setup above, then what would be the Setup?

Sorry for the inconvienience,

thanks,

[amit-singh]

AFAIK, only 1 vlan can be your native vlan. You cannot configure multiple native vlans on the switches.

HTH, Please rate if it does.

-amit singh

[Mohamed Sobair]

Hi Amit,

that't why i am concerned about the tagged vlans, and if u review up my previous Posts with Paresh u will find that I am trying to connect both Switches through trunk that can carries tagged vlan in order to be able connecting multiple links.

So Simply, I need to know what is the a vailablity of doing that since i tried the tagged vlans but the traffic is discarded for some reason.

Appreciate any feedback,

Mohamed

[Mohamed Sobair]

Guys,

I am puzzled regarding Solution for the a bove. Any updates....

your feedback is higly appreciated,

thanks