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New Member

vlan and dhcp

I have two vlans configured on a 2912 and a 2924 with an isl trunk connecting them. I have a 1605 router connected to the 2912 with one interface on a vlan 1 port and one interface on a vlan 2 port. PCs connected to ports on vlan 2 request an ip via DHCP. The DHCP server is located on vlan 1 with two different subnet pools. The router interface on vlan 2 is configured with an ip-helper address. Prior to a power fail on the weekend the DHCP server was handing out ip addresses to pc's on vlan 2 appropriately based on the subnet of the routers interface on vlan 2. For example vlan 1 is on subnet and vlan 2 is on Request from the router interface on should be getting ip from dhcp on that same subnet but they are getting them from the .10.0 subnet. I have cleared arp caches on all devices. Our DHCP config has been confirmed correct by the vendor. Why is the request from the router interface on subnet .20.0 getting ip's from the .10.0 address?


Re: vlan and dhcp

It almost sounds like you lost your VLAN config on the switches (making the VLAN2 PCs look like they're on VLAN1).

The router would be out of the picture at that point, since everything appears to be on the same network.

Try disconnecting the router from Either VLAN interface and see if the PCs still get their addresses (they shouldn't, if the router is passing the traffic).

If VLAN1 is the native VLAN, then even if the other switch was providing proper tag information, it would drop into the default VLAN and be passed on to the DHCP server.

Give it a shot. Take a look at the VLAN info on both switches, and the ISL trunk setup.

What frequently happens is that the configuration is up and running, but not saved. When the power goes out, the router / switch will revert to the last-saved config.

Good Luck


New Member

Re: vlan and dhcp

Startup and running configs are the same. I disconnected the router on vlan1 side of things and the pc's still get their address. So the router is not passing the traffic at all. So now I am really confused. Do we dump all vlan configs and rebuild?