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Community Member

DHCP server

I have created 3 vlans on a layer 3 switch (cisco 3560). The 3 vlans are able to talk by enabling interVlan routing. I have also a DHCP server on one of the vlan, but i don't khow how to enable the DHCP server publish ip addresses to devices sitting into the 2 other vlans.

any ideas?

5 REPLIES
Hall of Fame Super Gold

Re: DHCP server

walid

First make sure that the DHCP server has correct DHCP scopes configured for each subnet. Then on the VLAN interfaces of the switch that are not connected to the DHCP server configure this:

ip helper-address

So if you have 3 VLANs that are for example VLAN 2 and 4 and 6. And if the DHCP server is connected on VLAN 4, then end stations in VLAN 4 can communicate directly with the server and be assigned addresses. When you configure ip helper-address on the interfaces VLAN 2 and 6 then the switch will forward DHCP requests from VLAN 2 and 6 to the server in VLAN 4. And the server will be able to assign addresses in all of the subnets/VLANs.

HTH

Rick

Community Member

Re: DHCP server

it seems it's working. The devices from different vlans are getting ip addresses, but they are getting a different default-gateway. For example, a device on vlan 6 is getting an ip address within the subnet, but the default gateway is different than the ip address of vlan 6, and obviously i'm losing the connection.

anything to do with the switch?

Hall of Fame Super Gold

Re: DHCP server

walid

What address is being assigned as the default gateway for devices in vlan 6?

Perhaps you can post the configuration within the switch?

HTH

Rick

Community Member

Re: DHCP server

here is the configuration:

switch(config)#int vlan 4

switch(config-if)#ip address 10.1.40.1 2552.255.255.0

switch(config)#no shut

switch(config)#int vlan 5

switch(config-if)#ip address 10.1.50.1 255.255.255.0

switch(config)#no shut

switch(config)#int vlan 6

switch(config-if)#ip address 10.1.60.1 255.255.255.0

switch(config)#no shut

switch(config)#int vlan 5

switch(config-if)#ip helper-address 10.1.40.100

switch(config)#int vlan 6

switch(config-if)#ip helper-address 10.1.40.100

switch(config)#ip routing

switch(config)#exit

switch#

as you can see the DHCP server is on vlan 4 and has an ip address of 10.1.40.100

the server has published an ip address to a device on vlan 6 with

ip address: 10.1.60.25

default-gateway: 10.1.42.1

i'm expecting 10.1.60.1 for the default-gateway, and that's why once the ip address was published i lost the connection.

Hall of Fame Super Gold

Re: DHCP server

walid

My first guess would be that something is misconfigured in the DHCP server. I do not see anything so far that looks like a switch problem. The fact that the PC is getting assigned an IP address seems to show that the helper-address is working and that the PC is communicating with the DHCP server. Can you verify what is configured on the DHCP server for default gateway in the scope for vlan 6?

HTH

Rick

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