Howto configure a Mikrotik as central DHCP server with switches as DHCP relays

April 27, 2013

I’ve found many articles about how to configure a Mikrotik RouterOS as DHCP relay or as simple DHCP server, but I didn’t find an article about following setup:

  • central Mikrotik DHCP Server (e.g. in the data center)
  • multiple VLANs/subnets for clients which are connected via a Layer3 Switch (or even more hops) to the central data center

According to the Mikrotik Wiki and the described options it is possible but there is no example shown and everyone uses the setup wizard to configure a DHCP Server. I’ll get most people don’t know what happens in the background … I’ll show this the old fashioned way. 😉

Setup for this example/howto

For this example we assume that we’ve 4 VLANs with following subnets:

  • 10.88.0.0/24 (data center VLAN for the servers and the DHCP server)
  • 10.88.10.0/24 (clients network location 1)
  • 10.88.11.0/24 (clients network location 2)
  • 10.88.12.0/24 (clients network location 3)

The Mikrotik has the IP 10.88.0.100 and connected via ether1 to the data center VLAN. The Layer3 switches use always the .1 IP address in the clients networks.

Layer3 Switches / DHCP Relay

Most (if not all) switches which are able to perform at least simple layer 3 tasks (often also labeled as Layer2+ switches) are able to forward DHCP requests. Check the manual of the switch for this. One setting I came across sometimes leads to problems. It is called “DHCP Relay delay” and is sometimes set to 1 or 2 seconds in the the default configuration. This setting allows a local DHCP Server to answer faster, but sometimes (specially embedded clients) don’t wait that long and run into an error. If there is no local DHCP server set this timer to 0 seconds.

You’ll need to set the DHCP Server IP on the switch to the IP of the Mikrotik.

Mikrotik as DHCP Server

First we configure our pools of the client networks, the mikrotik will give out IPs from this ranges:

/ip pool
add name=poolClientsLocation1 ranges=10.88.10.10-10.88.10.250
add name=poolClientsLocation2 ranges=10.88.11.10-10.88.11.250
add name=poolClientsLocation3 ranges=10.88.12.10-10.88.12.250

Now we need to set the configuration the DHCP Server will handout the clients:

/ip dhcp-server network
add address=10.88.10.0/24 dns-server=10.88.0.100 gateway=10.88.10.1
add address=10.88.11.0/24 dns-server=10.88.0.100 gateway=10.88.11.1
add address=10.88.11.0/24 dns-server=10.88.0.100 gateway=10.88.12.1

And at last we configure which DHCP Relay gets which configuration/pool:

/ip dhcp-server
add address-pool=poolClientsLocation1 authoritative=yes disabled=no interface=ether1 lease-time=1w name=dhcpClientsLocation1 relay=10.88.10.1
add address-pool=poolClientsLocation2 authoritative=yes disabled=no interface=ether1 lease-time=1w name=dhcpClientsLocation2 relay=10.88.11.1
add address-pool=poolClientsLocation3 authoritative=yes disabled=no interface=ether1 lease-time=1w name=dhcpClientsLocation3 relay=10.88.12.1

This is all … 😉

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