Help with IP migrating on SVI

Matty Ray · ‎06-22-2016

Please bare with me. I'm a newbie. I'm a junior entry level engineer.

Migrating some devices from several smaller subnets /26 to a new /24 subnet. Senior engineers told me to have a plan to migrate these devices over since we won't be able to do this all in one night and test everything. So my suggested configuration would be temporary.

old subnets:

10.10.10.0/26

10.10.10.64/26

10.10.11.128/26

10.10.11.192/26

10.10.12.0/26

New subnet:

172.20.99.0/24

Would this work to allow both devices with old IP addressing and new ip addressing on this switch to communicate?

interface vlan 89

ip address 172.20.99.254 255.255.255.0

ip address 10.10.10.62 255.255.255.192

ip address 10.10.10.126 255.255.255.192

ip address 10.10.11.190 255.255.255.192

ip address 10.10.11.222 255.255.255.192

ip address 10.10.12.62 255.255.255.192

no shutdown

I've never done an IP migration from different subnets within the same VLAN so I need help please. Will the above configuration work? Senior engineers said that routing is all set on the core so my part is to come up with a plan to replicate IP addressing to allow the devices with both old IP and new IP to work on this switch within the same VLAN. Thank you.

Yad Singh · ‎06-23-2016

Hi Matty,

I read a long time ago that this is not a recommended design but this is possible. Your single VLAN will receive broadcast traffic for both subnets and it is not optimized. Broadcast will happen on L2 and both subnets will receive it.

Here is a discussion I came across:

https://supportforums.cisco.com/discussion/10885136/single-vlan-can-support-multiple-subnets

pwwiddicombe · ‎06-23-2016

You will have to add "secondary" to the end of all but the new primary IP address,

ip address 10.10.10.62 255.255.255.192 secondary

These are presumably already in your routing tables, if they are working today. Remember to add the NEW subnet to EIGRP or OSPF or whatever you use.

Do remember to remove all the old addresses on completion; once you have moved all clients (hopefully DHCP made that job easy !) and printers and other oddball/static addresses out of the old subnets. Some things work less than optimally when secondary addresses are used.