Configuring IP Helper to use multiple DHCP servers on the same VLAN

03/26/2020 33 17920

DESCRIPTION:

Because DHCP clients find their local DHCP server by broadcasting, it is usually considered important to only have one DHCP server per broadcast domain (VLAN), to avoid duplicate IP addresses and other conflicts.

However, only having one DHCP server means a single point of failure, so some networks are designed to use 2 or more DHCP servers on the same VLAN to achieve redundancy if one fails.

This can be achieved by 2 methods:

1. By carefully configuring the DHCP servers with address pools that are in the same subnet, but not overlapping.

2. By using Microsofts DHCP Failover

CAUSE:

This presents a problem for SonicWall's IP Helper. It does not allow multiple policies to be created with the same "From" address, so how can it forward clients DHCP traffic to a second DHCP server if the first one fails?

RESOLUTION:

1. Create an Address Object Group which contains the IP addresses of all the DHCP servers that are in the same VLAN.

2. Create an IP Helper DHCP policy as usual, but in the "To" field, instead of the IP address of a single DHCP server, put the Address Object Group created above:

Resolution for SonicOS 6.5

This release includes significant user interface changes and many new features that are different from the SonicOS 6.2 and earlier firmware. The below resolution is for customers using SonicOS 6.5 firmware.

1. Create an Address Object Group which contains the IP addresses of all the DHCP servers that are in the same VLAN.

2. Create an IP Helper DHCP policy as usual, but in the "To" field, instead of the IP address of a single DHCP server, put the Address Object Group created above: