Network Design - VLANS

[TimLaffan]

Hi All,

Abit of a open question really but am really afer abit of advise,

Currently we have a flat network 172.16.x.x 255.255.0.0 without any VLAN's, we are now looking to introduce VLAN's into the network using a /24 mask 172.16.10.x 255.255.255.x etc

My question which is the best way to-do this without causing major disruption to the network... All our access switches are 2950's with 2 stacked 3750's as the core\distribution. All switches have 2 1GB links back to the core using Ether Channel!

We have added the VLAN to the switches, and the next phase is to add SVI's, Ip helper addresses etc to the core, but we are unsure what down time this will cause -

Once thing that we have thought is to, split the /16 network into two, using 255.255.128.0, then phase the first group of IP's into /24 masks followed by the next group

Any advise would be great

[Nathan Spitzer]

I did this exact same thing once and you CAN do it with 0 downtime if you are willing to work a little ahead of time. What you do is map out all your default gateways ahead of time. Setup the existing layer 3 interface to have all the needed default gateways as secondaries.

Now, make sure you can ping all those IP's from the common VLAN then go to all your static devices (printers, etc) and change thier IP information to reflect whatever you've mapped them into (including the subnet mask). Since their default gateway IP exists as a secondary on the same VLAN they will still work and the L3 switch will "hair-pin" the traffic.

Since the hardest part is updating static devices like printers where queues have to be changed you can leasurely update all the hard stuff (printers, servers, etc). Then when you are ready you can create the VLANS, allocate them as needed to the proper ports and bounce all the interfaces with a "shut-no shut" and all the DHCP devices will work (assuming you've setup DHCP server correctly).

BTW: BEWARE OF FIREWALLS! They have long ARP caches of thier default gateway's, had this bite me.

Piece of cake and if done right the users never even know.

Nathan Spitzer

Sr. Network Communications Analyst

Lockheed Martin