Unanswered Question
Jan 31st, 2008

Hi Guys

Need some clarification on this. We have 2 subnets in a VLAN 10 e.g. and

Now we want to sepaarate out these vlans like in Vlan 10 and in Vlan 20.

But my thoughts is that for us to be able to physically configure this as

interface vlan 20

ip address

we will not be able to do this as this ip address range already exist in vlan 10. An error message will come up saying "network range already exist"

Am I correct in this? How best can we do this?

I have this problem too.
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lancehino Thu, 01/31/2008 - 08:52

You must first remove the address from vlan 10, then add the address to vlan 20

qwertys21 Thu, 01/31/2008 - 09:06

Does this mean, I will have to give the devices that have 2.2.2.x addresses in vlan 10 other ip addreeses so that i can pull out the 2.2.2.x addresses and put it in vlan 20?

Jon Marshall Thu, 01/31/2008 - 15:44


That would be a rather long winded way to do it to be honest.

You will need some downtime but not that much. You don't say which type of switches you are using but the "interface range.." command is useful for this. Using this command will allow you to allocate all the ports on switch to the new vlan. So depending on how many switches you have this could be done quite quickly ie.

1) Create vlan 20 at layer 2 and then create vlan 2 L3 interface.

2) remove address from vlan 10 interface and reassign to vlan 20

3) Use the "interface range... " command to allocate all the relevant ports per switch to the new vlan.

If you prepare the commands in a text file you can cut and paste with minimum downtime.



qwertys21 Fri, 02/01/2008 - 01:28

Thanks Jon

This is on a Cat 3750

What I really wanted to be clear about as I don't want any surprises is

If I have to do


Vlan 20 name ???


Switch A>

Int vlan 20

ip address 255.x.x.x

stand ip 255.x.x.x

Switch B

Int vlan 20

ip address 255.x.x.x

stand ip 255.x.x.x

and then do an

interface range fasteth 0/1-10

switch acc vlan 20

which is fine.

At the same time,, and are part of the 2.2.2.x/25 subnet which is still in Vlan 10 even though those 3 addresses are available and are not physically in use.

Will this cause a problem? As all I am doing in taking out 3 addresses from a although the subnet is currently in vlan 10.

Will this work?



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