====== Failover ======

Setting up failover is fairly straightforward. There are several guides and howtos on the Internet. I used the following:

ISC: [[https://kb.isc.org/docs/aa-00502|A Basic Guide to Configuring DHCP Failover]], and

Steven Diver: [[https://stevendiver.com/2020/02/21/isc-dhcp-failover-configuration/|ISC DHCP Failover Configuration]]

Here are the important sections of the respective configuration files. The Main Router has the IP address 192.168.19.1 and the Backup Router has 192.168.19.44.

===== Main Router (Nounours) =====

  # dhcpd.conf
  #
  # Configuration file for ISC dhcpd
  #
  # Configuration of failover
  #
  failover peer "failover-partner" {
     primary;
     address 192.168.19.1;
     peer address 192.168.19.44;
     max-response-delay 60;
     max-unacked-updates 10;
     mclt 3600;
     split 128;
     load balance max seconds 3;
  }
  .....
  # Subnet specification for unknown clients, primarily on the WLAN
  subnet 192.168.19.0 netmask 255.255.255.0 {
    pool {
      failover peer "failover-partner" ;
      range 192.168.19.241 192.168.19.253;
    }
    # .254 is reserved for the 'floating' IP address of VRRP
  }
  
  # Subnet specification for IOT devices on VLAN 2
  subnet 172.19.2.0 netmask 255.255.255.0 {
    pool {
      failover peer "failover-partner" ;
      range 172.19.2.10 172.19.2.99;
      option domain-name-servers 1.1.1.1;
      option ntp-servers 0.dk.pool.ntp.org;
      option ntp-servers 1.dk.pool.ntp.org;
    }
  }
  ......
  
===== Backup Router (Gate-rpi) =====

The configuration of the Backup Router is almost identical to the Main Router, the difference being the specification of the partner and it not having the line 'split 128'.

  # dhcpd.conf
  #
  # Configuration file for ISC dhcpd
  #
  # Configuration of failover
  #
  failover peer "failover-partner" {
     secondary;
     address 192.168.19.44;
     peer address 192.168.19.1;
     max-response-delay 60;
     max-unacked-updates 10;
     load balance max seconds 3;
   }
   ......