Can hosts in Native Vlan1 on two different subnets communicate to each others

[minagamal237]

Hi,

I have plenty of servers connected to three layer2 switches, all of them are connected to native Vlan 1 on these switches. They are all communicating purely layer2. (No SVI's in the switches).

I am planning to change the IP addresses of all of these servers to a new subnet.

My question is concerning the transition period, are the servers that got the new IP of the new subnet going to be able to communicate with the servers that are still in the old subnet? i.e. can a server with IP 10.x.x.x communicate with a server with IP 192.168.x.x where both servers are in Vlan1 ?

[Jon Marshall]

It really depends on the host OS and how or whether you can get it to arp for IPs that are not on the same subnet.

Basically a vlan is a broadcast domain and arp is a broadcast so when a device sends an arp request all other devices in the vlan, regardless of their IP address, will see the arp.  That is not the problem though.

The problem is getting a 192.168.x.x device to send an arp for a 10.x.x.x device because the 192.168.x.x device knows the other device is in a different subnet so it will know it cannot send an arp for that IP but instead must send an arp for it's default gateway but in your case you don't have this because it is all L2 and you don't have a L3 switch by the sounds of it.

So you need to "trick" the device into sending an arp for any other IPs whether they are on the same subnet or not. And this is very much OS dependant ie. nothing to do with the Cisco switches at all and not something you can setup using Cisco commands.

So it is really trial and error on the host devices I'm afraid.

Jon

[minagamal237]

Jon, thanks for your reply, I will do a test with one server and see how it goes. but thanks for confirming that nothing to be done on the switches.