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Interface VLAN question

Ok....forgive me for my ignorance, but I have a couple questions regarding VLANs that aren't making any sense to me.  Let me explain my scenario.

I have numerous remote sites.   Some of these have VOIP at their locations, but not all.    Basically, at the remote sites, we have a router (either 2821 or 1841) and a switch (35Xx Catalyst). 

Now here is where my questions begin.    At the VOIP sites, we set up a trunk between the switch and the routers Gig interface.   On the Gig interface of the router, we have numerous sub-interfaces to represent each VLAN traversing the trunk.    Now for the life of me, I do not understand why if I scroll down the config I see VLAN interfaces without any IP in them.  This is in the router.   Why would they be there?  Are they serving any purpose?    As long as my sub-interfaces are setup as encap dot1q trunks, shouldn't the sub-interface be acting as my layer 3?   I'm not sure I understand this.

I tried to configure one of my other routers to reflect one of the VOIP router configs....it doesnt even give me the option to create an interface VLAN.    I did notice that the 2821 that does have the Int VLAN created also has a 4 port FastE card in it.   Would this be why it's allowing me to create the Interface VLANS?

And therefore, this leads to my next questions,   Which is better practice?  To create an Interface VLAN on the switch and have it act as layer 3 or to create the trunk between the router and switch and use the router sub-interfaces as the layer 3 piece?  Which is more efficient? 

Sorry for all the random questions, but i'm struggling understanding why there are interface VLANS in my 2821 that doesnt have any IP addresses.  I just can't understand why they would need to be there.

Any information you can provide would greatly be appreciated.  

Thanks in advance,

2 ACCEPTED SOLUTIONS

Accepted Solutions

Re: Interface VLAN question

They are not random questions; they are actually very good questions.

The VLAN interfaces are almost certainly related to the 4-port card.  My guess is that it isn't just an ordinary 4-port card, but a 4-port mini-switch, probably called an HWIC-4ESW or something similar.  It is designed for a SOHO (Small-Office-Home-Office) environment, where 4-ports might be sufficient.  Now, I don't have any experience with that card, so I'm not sure how it links in with the rest of the router.

As for the 4 Ethernet ports, I presume you can either give them no switchport and use them as normal router Fa interfaces (if it will allow you to do so?), or you can make them switchports, assign them to a VLAN, and route in and out of the vlan interface.  But I don't think it will bridge automatically between the subinterfaces you have set up and the switch vlan interfaces.  If you have just one switch at the remote site, I would be inclined to connect it to the router via a trunk to Gi0/0, route to your subinterfaces, and ignore the 4ESW card.

Kevin Dorrell

Luxembourg

P.S. It may well be that the 4ESW will not allow you to do no switchport on its interfaces.  If that is the case, then the VLAN interfaces will be the only way you can route in and out of this interfaces.  The interfaces could also be useful for connecting anciliary equipment like firewalls, etc.

VIP Super Bronze

Re: Interface VLAN question

jonesl1 wrote:

Thank you both for your quick responses and attempting my brain to comprehend this.

You are correct in your assumption....it is an HWIC-4ESW card.  

So....with that said....if i'm running my vlan traffic through the trunk to the gig interface of my router, there really shouldn't be any reason

that I should have the following commands in the router.  Does this sound correct?

Int vlan 10

no ip address

int vlan 20

no ip address


You are correct if you do not have any users connected to these ports/vlans.  Again if no user is connected to these ports then you can delete these vlans to minimize clutter.

9 REPLIES
VIP Super Bronze

Re: Interface VLAN question

Hi,

No, that is not true.  Here is a config on a 2821 with on board gi0/0 with sub-interface

System image file is "flash:/c2800nm-adventerprisek9_ivs-mz.124-3g.bin

interface GigabitEthernet0/0.1
encapsulation dot1Q 30
ip address 30.30.30.1 255.255.255.0
no snmp trap link-status
end

What version of IOS are you running?

HTH

Reza

New Member

Re: Interface VLAN question

On the 2821's we are running 12.4(9)T7

On the 1841's we are running 12.4(25)

VIP Super Bronze

Re: Interface VLAN question

You should be able to create sub-interfaces with these images.  What is the error message when you try to create a sub-interface? can you post it from the router with the error message?

New Member

Re: Interface VLAN question

I may need to explain myself more.  When i say interface vlans..i mean:

Int Vlan 10

  no ip address

int Vlan 20

  no ip address

My sub-interfaces look fine, i just cant understand why the lines above in italics are in my config and what purpose they are serving.

VIP Super Bronze

Re: Interface VLAN question

You have thes vlan interface, because you have a switch module in you router.  I have for example 3845 with 16 port switch module and that is where you create layer-2 vlans just like the one you have in your config.  What modules do you installed in that router?

Re: Interface VLAN question

They are not random questions; they are actually very good questions.

The VLAN interfaces are almost certainly related to the 4-port card.  My guess is that it isn't just an ordinary 4-port card, but a 4-port mini-switch, probably called an HWIC-4ESW or something similar.  It is designed for a SOHO (Small-Office-Home-Office) environment, where 4-ports might be sufficient.  Now, I don't have any experience with that card, so I'm not sure how it links in with the rest of the router.

As for the 4 Ethernet ports, I presume you can either give them no switchport and use them as normal router Fa interfaces (if it will allow you to do so?), or you can make them switchports, assign them to a VLAN, and route in and out of the vlan interface.  But I don't think it will bridge automatically between the subinterfaces you have set up and the switch vlan interfaces.  If you have just one switch at the remote site, I would be inclined to connect it to the router via a trunk to Gi0/0, route to your subinterfaces, and ignore the 4ESW card.

Kevin Dorrell

Luxembourg

P.S. It may well be that the 4ESW will not allow you to do no switchport on its interfaces.  If that is the case, then the VLAN interfaces will be the only way you can route in and out of this interfaces.  The interfaces could also be useful for connecting anciliary equipment like firewalls, etc.

New Member

Re: Interface VLAN question

Thank you both for your quick responses and attempting my brain to comprehend this.

You are correct in your assumption....it is an HWIC-4ESW card.  

So....with that said....if i'm running my vlan traffic through the trunk to the gig interface of my router, there really shouldn't be any reason

that I should have the following commands in the router.  Does this sound correct?

Int vlan 10

no ip address

int vlan 20

no ip address

VIP Super Bronze

Re: Interface VLAN question

jonesl1 wrote:

Thank you both for your quick responses and attempting my brain to comprehend this.

You are correct in your assumption....it is an HWIC-4ESW card.  

So....with that said....if i'm running my vlan traffic through the trunk to the gig interface of my router, there really shouldn't be any reason

that I should have the following commands in the router.  Does this sound correct?

Int vlan 10

no ip address

int vlan 20

no ip address


You are correct if you do not have any users connected to these ports/vlans.  Again if no user is connected to these ports then you can delete these vlans to minimize clutter.

New Member

Re: Interface VLAN question

You guys are both awesome!   Thank you so much for your help!   My brain also thanks you!

It makes so much more sense now. 

Again, thanks to both of you!

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