How to expand an existing subnet in a bridge domain

[jdevyor]

Hello.  I have an existing /24 subnet running in a production bridge domain.  I would like to expand this subnet to a /23 and I am looking for the least intusive/no outage option.  I know I can add another subnet to a bridge domain, but I want to keep the same gateway address as the existing /24 subnet.

Is it an option to create a new /23 subnet in the same bridge domain and use the same gateway address as the /24 and not choose the option "Make this IP address primary" just to create the new subnet and then plan a cut where I would disable the "Make this IP address primary" on existing /24, enable that same option on the new /23, test pinging, and then delete the /24 subnet from the bridge domain? 

[RedNectar · Accepted solution]

Hi @jdevyor ,

Changing the mask of the subnet is the least of your worries. That is a trivial task compared to changing the mask of all the existing devices that use the /24 mask on the subnet already.

But that said, you can just add the /23 IP address to the subnet, then delete the /24 - providing your /23 doesn't overlap with another subnet, then delete the /24 subnet

Step 1:

Step 2:

Since your BD will keep the same MAC address, you should see no disruption - although if the subnet is advertised externally, there will be some convergence time.

[jdevyor]

Thanks. Your steps worked just fine. Was pinging the gateway the whole time and didn’t miss a ping. Devices in the subnet are all on DHCP and all went well.

[RedNectar]

Hi @jdevyor ,

Glad it worked for you. And if DHCP kept working too, that's a bonus.

Trivial point - but you've marked you reply as the correct answer rather than mine - although it won't matter because surely anyone who finds this thread will figure it out.

[jdevyor]

I corrected it.  Sorry, newbie.