Multiple DHCP Pools in Ubuntu Server with DHCP Relay

Question

I am currently using Cisco SG-350 Series Switch as the DHCP server on the network. I was having 16 VLANs, and my SG350-28 switch was acting as the DHCP server for all the VLANs. All was running fine until I added a few more VLANs and corresponding DHCP Server pools. The issue is, it supports only up to 16 DHCP pools. Whereas I need more DHCP pools.

Is it possible to have an Ubuntu server as the centralized DHCP server on the network for all the VLANs and can I have my SG350-28 switch use the DHCP Relay for each VLAN?

Answer 1

As per your Description You are using SG350 switch as your core switch. Therefore it is not a good idea to Configure DHCP pool in this switch because it make additional overhead on your switch. In addition to that SG350 switch has limited feature for redundancy and therefore if there is a failure you will lost everything.

You can use separate DHCP pool such as ubuntu, windows or separate router such as netgear....etc. But i don't recommend to use DHCP pools on separate locations. You can move to all DHCP pools to separate DHCP pools. Otherwise It will generate additional complexity on your network.

Answer 2

The solution to my problem was quite simple, and I further complicated it by thinking more about it.

While configuring the ISC DHCP server on Linux, you can create classes for each subnet in the DHCP pool. Each class would match the 'giaddr' field in the DHCP requests and this would be the IP address of the DHCP relay agent. Within the subnet definition, match the appropriate class.

Below is the sample configuration.

  subnet 10.0.0.0 netmask 255.255.255.0 { }

  class "VLAN1" {
          match if (binary-to-ascii(10,8, ".", packet(24,4)) ="10.0.1.1");
  }

  class "VLAN2" {
          match if (binary-to-ascii(10,8, ".", packet(24,4)) ="10.0.2.1");
  }

  class "VLAN3" {
           match if (binary-to-ascii(10,8, ".", packet(24,4)) ="10.0.3.1");     
  }

  subnet 10.0.1.0 netmask 255.255.255.0 {
          pool {
                  allow members of                "VLAN1";
                  default-lease-time              86400;
                  max-lease-time                  7200;
                  range                           10.0.1.10 10.0.1.254;
                  option routers                  10.0.1.1;
                  option broadcast-address        10.0.1.255;
                  option subnet-mask              255.255.255.0;
                  option domain-name-servers      4.2.2.2;
  }
  }

  subnet 10.0.2.0 netmask 255.255.255.0 {
          pool {
                  allow members of                "VLAN2";
                  default-lease-time              86400;
                  max-lease-time                  7200;
                  range                           10.0.2.10 10.0.2.254;
                  option routers                  10.0.2.1;
                  option broadcast-address        10.0.2.255;
                  option subnet-mask              255.255.255.0;
                  option domain-name-servers      4.2.2.2;
  }
  }

  subnet 10.0.3.0 netmask 255.255.255.0 {
          pool {
                  allow members of                "VLAN3";
                  default-lease-time              86400;
                  max-lease-time                  7200;
                  range                           10.0.3.10 10.0.3.254;
                  option routers                  10.0.3.1;
                  option broadcast-address        10.0.3.255;
                  option subnet-mask              255.255.255.0;
                  option domain-name-servers      4.2.2.2;
  }
  }