Moving default route from one layer 3 switch to another

[CWK_Rob]

I'm attempting to replace a MS300 layer 3 switch with a new MS350X. I have both switches online in the network and my next step would be to move all of the Interfaces and Static Routes to the MS350X.

I'm wondering about the transition when I change the default route. Right now, the MS300 handles the default route and the site is connected via Air Fibre using a transit network. Can I move the default route to the MS350X and still have the Air Fibre patched to the MS300 until I am ready to remove the MS300 and move the patch cable? Will everything still communicate with the cloud if the MS350X holds the logic for the default route while the MS300 is connected to the Air Fibre uplink?

[PhilipDAth]

If their is a trunk connection between the MS300 and MS350 allowing all VLANs then it would work.

I've never heard of an MS300, so this probably wont work, but you could take a look at using switch cloning.

https://documentation.meraki.com/MS/Other_Topics/Switch_Cloning 

If that worked you would clone the config from the old switch to the new, and then remove the layer 3 config from the old switch.

[CWK_Rob]

 PhilipDAth: Sorry, I meant MS320.

Yes, there is a trunk connection between the MS320 and MS350X allowing all VLANs, so I do think it will work as well.  I just wanted to be sure before i went ahead and moved the routing config.  It's a public-use facility.

I will also look into switch cloning.

Cheers

[PhilipDAth]

Note the switch cloning method would definately result in an outage.

Also note that when you move the default gateway from one layer 3 switch to another (assuming you are keeping the same IP address) that the clients will loose connectivity because they will have cached the ARP entry for the old switch.  Typically ARP cache times are from 10 minutes to 4 hours.

Because of this problem I often reboot the access switches where devices are plugged in (dropping their link and bringing it up again makes them ARP again).  You can also manually unplug and re-plug them in.  You can also reboot the hosts, or manually clear the host ARP cache.

[CWK_Rob]

I just looked at switch cloning.  It's not an option in this case, switch models have to be the same.

Thanks for the tips with the ARP cache, something to watch out for.

[CWK_Rob · Accepted solution]

So, I had the exact scenario last night, but at a less critical, less public facing area of my network. 

Once both layer 3 switches are in place, with the new one connected to a trunk port on the original, all interfaces and static routes (logic) moved to the new switch without issue.  I did try to just move the interfaces at first, but Meraki Dashboard forced me to move all routing logic to the new switch, not piece by piece as I intended.  There was no service interruption, however.

After I was comfortable with the routing, I replaced the original layer 3 by moving it's uplink to the new layer 3.  This re-patch had a very short outage, as expected.