samirshaikh52 wrote:
Hi Jon,
Thanks for your notes.
But let me tell you something that I have been already cleared everything and in one my previous post.
-I have 2 vlans configured on layer 3 (6500)switch for eg. vlan 10 and vlan 20
- vlan 10 subnet 10.1.10.0/24
-vlan 20 siubnet 10.1.20.0/24
- below is the vlans configuration
interface vlan 10
ip address 10.1.10.1 255.255.255.0
interface vlan 20
ip address 10.1.20.1 255.255.255.0
- further i have adsl modem. dsl has default configuration with dhcp service enabled
modem subnet: 192.168.1.0/ 24, dhcp range: 192.168.1.100-192.168.1.254
- this modem is connected to access switch (3560) on the port and this switchport is configured on vlan 10
- the clients in vlan are sucessfull able to get the dhcp ip from this modem and they browse internet
- client in vlan are getting folowing ip:
IP: 192.168.1.101
SM: 255.255.255.0
GW: 192.168.1.1
DNS: 192.168.1.1
If you had any question here pleas let me know.
Okay, so the modem is connected to a port in vlan 10 ie. the subnet 10.1.10.0/24. So what is the modem IP address ie. it has to have an IP address from the 10.1.10.x subnet range ?
This is where the confusion arrises from i think. You say that the modem is handing out IP's for the 192.168.1.0/24 subnet. And that it is connected to vlan 10 on the switch. And that clients in vlan 10 are getting IPs in the 192.168.1.x range with 192.168.1.1 (presumably the modem) acting as the gateway. This works for clients in vlan 10 because they are broadcasting their DHCP request and are receiving an answer from the modem. But it can't work for any other vlans because how does the switch communicate with the modem ie. the modem has 192.168.1.1 address but the switch thinks that vlan 10 subnet is 10.1.10.x.
So if you want any other clients to receive an IP address from the modem you will need to -
1) setup another DHCP pool on the modem - can this be done ? because the modem may only be able to hand out addresses for the subnet it's interface is in ie. 192.168.1.x/24
2) critically though you must make the modem routable from the 3560 and it isn't because the 3560 thinks that the vlan the modem is in is 10.1.10.x. So you either need to either -
a) change the vlan 10 ip address to be an address from the 192.168.1.x subnet which assumes there are no other clients in vlan that have 10.1.10.x addressing ie. they all get their address from the modem
or
b) you need to create a new vlan and vlan interface on your 3560 switch eg. an unused vlan -
3560(config)# vlan 40
3560(config)# int vlan 40
3560(config-if)# ip address x.x.x.x <--- where x.x.x.x is an address from the 192.168.1.x range
then you need to allocate the modem and all the clients in vlan 10 that you want to get a 192.168.1.x address to vlan 40
If you do either a) or b) above then you can then use 192.168.1.1 as the "ip helper-address" under the other vlan interfaces and it should work.
Having said all that if the modem cannot support multiple DHCP pools then neither a) or b) will work for any of your other vlans ie. they won't be able to get an address from the modem. In this case you need something else to setup up the DHCP pools on. Personally i'm in favour of using an OS such as windows or unix to do this but if you can't do that you can setup DHCP pools on the 3560.
Regardless of how you setup the pools you really need to sort out that modem to vlan 10 issue.
Please come back if you need clarification on anything. Key thing at the moment is to find out whether your modem supports multiple DHCP pools.
Jon
