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Cisco VLAN Setup, a few questions.

Posted on 2006-05-30
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Last Modified: 2008-01-09
Howdy folks,

I'm trying to get my head around a few things.  Firstly I understand that if you have say 3 switches with 3 vlans you can use a router and a trunk link to route between these vlans.  With this you need just one interface correct ?

Now I'm trying to work out, say I have one LAN with 3 VLANS, for arguments sakes VLAN 1 is my management vlan, VLAN 2 is my sales VLAN and VLAN 3 is my support vlan.  I am using trunking on one of my routers fast ethernet port to route between these happy as larry.  

Now say on my other fast ethernet port of the same router I have the same setup, exactly the same VLAN numbers etc.  Is it possible to send VLAN information from one network to another using the router or does this vlan info have only LAN significance rather than WAN significance ?

A bit of an odd question (I hope it makes sense) if not I can try and clarify but just let me know what you think.  

Cheers folks ...

- Jamie



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Question by:Jase_x
8 Comments
 
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Expert Comment

by:td_miles
ID: 16794374
so you are essentially wanting to do VLAN trunking across a WAN link, is that the question ?
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Author Comment

by:Jase_x
ID: 16794402
Well, Im passing the trunk information from my routers f.ethernet 0/0 port to the same routers f.ethernet 0/1 port if that helps.

- Jamie  
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by:td_miles
ID: 16794430
You could probably do it, but the question is WHY ? If you have devices on the same VLAN then they are on the same broadcast segment, do you really want to extend this across a typical WAN link ? If your WAN link is instead a 100Mb (or even Gb) ethernet link, then just get rid of the router and plug the WAN link into a switch at either end and just pretend that the two switches are right next to each other and configure VLAN's as per normal.
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Author Comment

by:Jase_x
ID: 16794449
I was just curious as to whether it was possible and how it was done.  Just curious more than anything.  How would you go about configuring this ? I just wanted to see it in action.

- Jamie
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Expert Comment

by:Don Johnston
ID: 16794828
You could do it, but not through a router.

By defination, routers move traffic between different networks. Since a VLAN and an network are the same thing, you would end up with two interfaces on the router connected to the same network. As soon as you tried assigning the IP address on the second interface (or subinterface in this scenario), you would get a message that the IP address "overlaps" with the IP address on the other interface.

The only way to get it to work would be to disable routing and enable bridging on the two interfaces.
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Expert Comment

by:td_miles
ID: 16794945
I'm sure it could be done somehow, but not sure how you would go about doing it. Sorry I can't help :(

MPLS is used on WAN connections to perform a similar function (ie segregate traffic)

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Accepted Solution

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lrmoore earned 200 total points
ID: 16797574
You cannot do it.
As Don pointed out, you cannot assign two interfaces to the same Ip subnet. VLAN tagging does not carry through the router. If two router interfaces are in the same VLAN and both connected to the switch, the router would never see any traffic that was not destined for it exclusively because the switch would keep it "in-house".

Example:
\\ interface fast 0/0 is connected to switch trunk port Gig0/0/1
 interface fast 0/0
  descript management vlan1
  ip add 192.168.168.1 255.255.255.0
 interface fast 0/0.1
  encap dot1q 2
  descript VLAN2 Sales
  ip add 192.168.202.1 255.255.255.0
 interface fast 0/0.2
  encap dot1q 3
  descript VLAN3 Support
  ip add 192.168.203.1 255.255.255.0

>on my other fast ethernet port of the same router I have the same setup, exactly the same VLAN numbers etc.  
interface fast 0/1
  descript management
  ip add 192.168.168.2 255.255.255.0  <== no can do!
interface fast 0/1.1
  encap dot1q 2
  ip add 192.168.202.2 255.255.255.0 <== no can do!
interface fast 0/1.2
  encap dot1q 3
  ip add 192.168.203.2 255.255.255.0 <== no can do!

Interface fast 0/1 connected to what? Yet Another switch with trunk port? It must be. If so . .
>this vlan info have only LAN significance rather than WAN significance ?
Exactly. It only  has significance to the switchport interface that this router interface is connected to.
NO VLAN data is passed between fast 0/0 and fast 0/1. You can see that the IP addressing is already a problem.
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Author Comment

by:Jase_x
ID: 16797624
Thanks for the information, exactly the answer i was looking for.  Thanks for curing my curiosity :)

- Jamie
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