VLAN Routing.

Unanswered Question
Feb 1st, 2008
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Hi all.

Now, I know it sounds ridiculous, but I still wanted to try this out.

I assigned IP addresses in a dis-contiguous manner, and assigned them to different VLANs. I.e.

VLAN 100

VLAN 110

Now, I am using a 2924XL switch, and a 2811 router. I tried creating sub-interfaces on the Ethernet ports and assigning IP addresses, but that does not accept the IP as it says it is overlapping.

How then do I ensure traffic between randomly assigned IP addresses in different VLANs? Of course, I know if they are in the same VLAN, then they would be able to communicate.

Any understandings would be highly appreciated.

Kind regards.

PS: I am just trying it out. This style of addresses makes no sense whatsoever.

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Overall Rating: 3.3 (3 ratings)
Kevin Dorrell Fri, 02/01/2008 - 04:28
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The short answer is that you cannot do that. Each VLAN corresponds to an IP subnet. This is so that the router can say "this subnet goes out on this VLAN, that subnet goes out on that VLAN".

The long answer is that you can do it using a feature called "IP mobility". There is a lite version of IP mobility, and there is the full works. The challenge is to find out about it in the documentation.

Kevin Dorrell


glen.grant Fri, 02/01/2008 - 04:34
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The addresses are all in the same subnet and you cannot do that . You route between subnets. Say for vlan 100 you have address range 192.168.1.x/24 , then you would put a different subnet on another subinterface say vlan 110 192.168.110.x/24 . If your switch is capable of trunking then the link to the switch will be a trunk carrying vlan 100 and 110 . You assign the different ports on the switch into the correct vlans and you should be able to route between them , but you cannot put individual addresses on the interfaces like you are trying to do . All users within a givin address range must be in the same vlan.

Rick Morris Fri, 02/01/2008 - 11:37
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When you create the sub-interfaces on the router you must not have any sub in the same network, each sub-interface must have its own subnet.

For example:

FastEthernet 0/0.1

ip address


FastEthernet 0/0.2

ip address


FastEthernet 0/0.3

ip address

See the issue. You cannot have something like this:

FastEthernet 0/0.1

ip address


FastEthernet 0/0.2

ip address

This will give you an error because that IP block is already assigned to 0/0.1

So if you wanted to statically assign IP's in a particular VLAN then you need to make sure that you have #1 a large enough block in that network range, and #2 assigned the correct IP in the right VLAN


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