I came across an issue when I configure DHCP scope on multiple VLANs with /26 subnet mask.

Platform is ClearOS 6.6.1 gateway mode on vsphere 6.0 VM + Cisco 3560C + Cisco 1142 autonomous AP

I configured 2 VLANs with DHCP scopes in ClearOS, 10.21.8.64/26, 10.21.8.192/26, all terminated on ClearOS(router-on-a-stick). Trunk to Cisco 3560 and 1142 AP(two SSIDs in two different VLAN).

So I have:

VLAN20: 10.21.8.64/26
Gateway: 10.21.8.65
DHCP scope: 10.21.8.70-10.21.8.80
lease expire: 12 hours

VLAN30: 10.21.8.192/26
Gateway: 10.21.8.193
DHCP scope: 10.21.8.200-10.21.8.210
lease expire: 12 hours

Everything was fine when client connected to the LAN initially. desktop/Laptop/iPhone/Android all got their IP addresses in right VLAN.

When I disconnected the cable and connected to another VLAN, interesting thing happened. the client remained the previous IP address, only changed the gateway.

For instance, a laptop previously connected to 3560 fa0/1 which is in VLAN20 and got an IP address 10.21.8.73 with gateway 10.21.8.65.

I disconnected it and connected to 3560 fa0/2 which is in VLAN30, it remained the IP address unchanged(still 10.21.8.73) and got gateway 10.21.8.193.

the laptop couldn't access Internet because of wrong IP address it got.

Looking at the DHCP server page, client lease page shows the laptop is still on previous IP address. Unless I delete it and renew the IP address manually on client, which doesn't make any sense.

I have tried on wireless clients as well(Laptop/iPhone), same issue happened, when I switch between two SSIDs, one of SSIDs doesn't have Internet access.

Just wondering if anyone has the same issue? Thanks!