Changing default VLAN from /28 to /26 - will it cause an outage?

[CWK_Rob]

Hi All,

I've got a fairly simple network expansion coming up.  However, all of my switches are on VLAN 1.  I inherited the network with a /28 subnet for VLAN 1, limiting my number of devices. 

I'm wondering if I make a change from /28 to /26 in the Addressing and Vlans menu, will it cause a recalculation outage?  My instinct says yes, it will, but I want to ask you fine folks.

My other option would be to create a management vlan (not present as of now) and add my new switches to that.  Then, move the other over in a controlled manner.

Thoughts?

[RaphaelL]

There are no simple answers. 

Yes and No. I would tend to say 'no' but it depends on your current setup. Vaguely what I would watch for :

Is there a dynamic routing protocol that has learned the /28 ?

Is there any firewall rules based on the network /28 ? You will need to change the subnet to /26 also in the FW rules

Is there any devices configured with static IPs ? ( You will need to change the network mask and/or the default gateway on the devices ) 

Is there any DHCP pool related to that /28 ?

I'm pretty sure that the DHCP hosts on that subnet will need to renew their leases to obtain the updated network information ( net mask , default gateway )

[GIdenJoe]

If expanding your network does not cause you to overlap into other networkspace in your current routing table you should not have any issues because you don't have any devices outside the current scope.  If you however move the gateway outside of the boundary of the current /28 you will temporarily lose routing.

So if you have to move your local router outside of the bounds of the old /28 range I would first config the switches to the new subnetsize and gateway and afterwards change your router so connectivity is restored.

However adding a separate VLAN for management is better because switching itself won't stop working even when you flip from one mgmt network to another.  I always have a separate VLAN for switches and AP's.

[Mike6116]

It will be better an easier to create a new mgmt vlan and add new siwtches to it,and then move the old switches to the new vlan, this is fairly easy to do and no downtime at all

[CWK_Rob]

Because my gateway was at .1 of the /28 network, it caused no outage to move to a /26.

All good, time to move forward with the rest of the config!