Vlanning, need more help.

Answered Question
Feb 1st, 2008

Hi All

We have a Vlan 10 which has 2 subnets 1.1.1.0/24 and 2.2.2.0/25 which is alive, up and running.

Now we want to divide out the vlans into 2 subnets namely

vlan 10 - 1.1.1.0/24 and

vlan 20 - 2.2.2.0/25

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

L2

Vlan 20 name ???

L3

Switch A>

Int vlan 20

ip address 2.2.2.10 255.x.x.x

stand ip 2.2.2.100 255.x.x.x

Switch B

Int vlan 20

ip address 2.2.2.11 255.x.x.x

stand ip 2.2.2.100 255.x.x.x

and then do an

interface range fasteth 0/1-10

switch acc vlan 20

which is fine.

At the same time, 2.2.2.10, 2.2.2.11 and 2.2.2.200 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 2.2.2.0/25 although the subnet is currently in vlan 10.

Will this work? Won't it complain that there are duplicate addresses even though the ip addresses are not physically assigned but the subnet is still part of vlan 10.

Thanks

I have this problem too.
0 votes
Correct Answer by Jon Marshall about 8 years 10 months ago

Hi

Sorry, got sidetracked, should have answered your previous post.

You are right, you cannot do this as you already have an address out of this subnet allocated to vlan 10.

Not sure you would want to either, because presumkably some of your devices on vlan 10 are using the secondary 2.2.2.x address as their default-gateway so if you used different 2.2.2.x addresses on vlan 20 you would need to update the default-gateways on all your machines using the 2.2.2.x range.

I think you need to accept there will be some downtime and plan accordingly.

HTH

Jon

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Correct Answer
Jon Marshall Fri, 02/01/2008 - 03:13

Hi

Sorry, got sidetracked, should have answered your previous post.

You are right, you cannot do this as you already have an address out of this subnet allocated to vlan 10.

Not sure you would want to either, because presumkably some of your devices on vlan 10 are using the secondary 2.2.2.x address as their default-gateway so if you used different 2.2.2.x addresses on vlan 20 you would need to update the default-gateways on all your machines using the 2.2.2.x range.

I think you need to accept there will be some downtime and plan accordingly.

HTH

Jon

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