Hello Ciscosianer,
the change that you want to perform cannot be made without impact, you need a maintanance window to do the job.
First of all, you need to locate in your campus all the end users that have IP addresses in the IP subnet 10.20.10.x.
To get this information you need to use:
show arp | inc 10.20.10
take the output and put it in a text editor, that is the list of current alive hosts in 10.20.10.0/24 in your network with their corresponding MAC address.
For each MAC address that appears in the previuos list make a search in your campus using:
show mac address-table address <address>
where <address> is an item of the previous list.
You perform the command on the core switch and you check the port that is provided in the output.
The port can be the port to which the end user is connected or more probably a trunk port to an access switch.
You can use show cdp nei detail to find out the management IP address of that access switch
You connect to the access switch and you repeat the command
show mac address-table address <address>
again depending on the structure of your campus you end up finding the end user port or a port leading to another switch. Let's suppose you find an access port currently in vlan 404, it will need to be moved to new vlan 1404 so you take a note of this switch and port for the future.
Repeat all the previous steps for all the MAC addresses listed in the original show arp | inc 10.20.10.
You now have the list of the access ports that need to be moved from vlan 404 to vlan 1404.
You can also build a list of inter-switch trunk links that will need to carry the new vlan 1404.
At this point in a maintenance window you can:
a) create L2 vlan 1404 in a vtp server (if you are using VTP otherwise you need to create manually vlan 1404 on all switches if they are in VTP transparent mode)
b) make vlan 1404 allowed in inter switch links that lead from core to access switches with interested access ports (ports that have to be moved from vlan404 to vlan1404) Use
switchport trunk allowed vlan add 1404
on both ends of the trunk link (both core side and access layer side)
c) you delete the lines about IP secondary from SVI vlan404 and create interface vlan1404 with appropriate IP address and HSRP configuration
d) You move all the access ports that you have found previuosly with users in subnet 10.20.10/24 from vlan 404 to vlan 1404
e) You verify correct connectivity between hosts and SVI with ping tests, you ping also the HSRP VIP from the hosts
f) You may need to add the new L3 interface SVI 1404 to your routing protocol configuration to make the associated subnet advertised (if you are using a routing protocol)
check connectivity to/from other IP subnets in the campus to the 10.20.10/24
I'm sorry for the long post, but I hope I have given an idea of the work you need to perform
Hope to help
Giuseppe
