Tuesday, September 10, 2013

DHCP superscope


DHCP superscopes is in simple terms a logical grouping of DHCP Scopes. They  are used in scenarios where there are multiple subnets created in a particular Vlan. In this case, your Vlan configuration would look like this:

Interface vlan 107
ip address 10.120.12.1/24
ip address 10.120.13.1/24 secondary
ip address 10.120.14.1/24 secondary

Create scopes for all the above subnets in your DHCP , then create a superscope and add the scopes to it.

The ideal case is to have one subnet per Vlan and to create individual scopes in DHCP for these Vlans. You will have to configure IP helper address for these Vlans and point them to your DHCP IP address so that the clients in various subnets get IPs from the DHCP. Your Vlan configuration would look like this (assume that the Ip of your DHCP is 10.120.12.3)

vlan 12
interface vlan12 ip address 10.120.12.1/24
vlan 13
interface vlan13 ip address 10.120.13.1/24
ip helper-address 10.120.12.3
vlan14
interface vlan14 ip address 10.120.14.1/24
ip helper-address 10.120.12.3

Here we have created a virtual interface (at layer 3), which can do inter-VLAN routing. DHCp requests for a Vlan received at the virtual interface is forwarded to the DHCP server 10.120.12.3, after changing the giaddr to the interface IP. DHCP when it receives the request, compares the subnet from the interface with the scopes configures. When it finds a match, the IP allocation process is initiated



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