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IP Helper – single DHCP server with more scopes in remote network
One of the first thing during designing campus network is an IP addressation scheme and its assigning at Access Layer. For handing out IP addresses responsible is DHCP protocol, proper deployment of DHCP server in large environment can spare us a lot of trouble and issues.
Imagine you’ve got a network with 20 subnets and VLANS and DHCP. Does it mean you have to deploy 20 DHCP servers ? Of course not 🙂
Firstly let’s recall how DHCP works based on simple client-server diagram:
The communication between the Client and the Server proceeds in this way: the Client sends broadcast DHCP Discover packet in order to find DHCP Server, Server sends an offer of an IP address, the Client confirm its with DHCP Request and the Server sends ACKowledgement packet. Of course Client sends broadcast packets only.
Now, let’s talk about IP Helper. As I mentioned earlier the Client sends broadcast packets, but thanks to IP Helper and pointed by us L3 interface within the same subnet as the Client (in our case it will be SVIs) , IP Helper re-encapsulates broadcast packet to unicast packet, as we know unicast packet can be routed. IP Helper feature can forward not only DHCP packets but TFTP, DNS and time as well.
1. My DHCP server is Windows 2012, I’ve created 2 scopes for 2 Vlans 10 and 20 as you see on the below picture:
2. I used a static routing betwen the routers interfaces and between Switch interface g0/2 and R2 f0/0 interface
3. I’ve created Vlan 10 and 20 and assigned to them interfaces G0/0 and G0/1 respectively
4.I’ve run ‘ip routing‘ on the Switch and created Vlans L3 interfaces in order to run inter vlan routing:
Interface Vlan 10
ip address 192.168.10.1 255.255.255.0
Interface Vlan 20
ip address 192.168.20.1 255.255.255.0
5. the last part of configuration is using ip helper command with our Win2012 DHCP server under Vlan L3 interfaces
Interface Vlan 10
ip helper-address 10.0.0.1
Interface Vlan 20
ip helper-address 10.0.0.1