Obsessive DHCP Server Deassigned/Assigned in Log

RouterOS Beginner Basics
1

I have two devices:

  • Mikrotik router.
  • Mikrotik AP.

Both of these devices are connected with an Ethernet cable.

The config of the Mikrotik Router:

  • Bridge has 4 ports.
  • Has DHCP Server on bridge.

The config of the Mikrotik AP:

  • All ports are associated to the bridge.
  • Doesn’t have DHCP Server.
  • Has only DHCP Client on its bridge.

For some reason the log of Mikrotik Router shows every minute or so:

DHCP_Router deassigned for

DHCP_Router assigned for

Lease time of DHCP Server in Mikrotik Router is configured to be 1 day for the Mikrotik AP.

In Wireshark I see the following flow:

  • DHCP Discover, by Mikrotik AP.
  • DHCP offer, by Mikrotik Router.
  • DHCP Request, by Mikrotik AP.
  • DHCP ACK, by Mikrotik Router.

Any idea what could cause this and how to fix?

2

Does AP show anything in logs? Does it have its DHCP-leased IP address assigned to bridge?

And those offers/acks, are they about same IP address?

3

Hey @mkx

AP barely has anything in its log.
Only 2 DHCP related log records were found in AP Log:

dhcp-client on bridge lost IP address <IP_1> - received NAK from dhcp server <IP_Router>
dhcp-client on bridge got IP address <IP_2>

The AP has this config:

  • ether1 is connected directly (via Ethernet) to Router
    ether1 is slave of bridge.
  • bridge has these interfaces:
    ether1 to ether5 and two wifi interfaces.
  • DHCP Client is configured on bridge.

In IP->Addresses, bridge has a different IP address than ether1 (though network is identical).
I'm not sure why, though I'm suspecting this.
IP address of by DHCP Client is the same IP as bridge.

Those Offers/Acks have the same IP address (which was offerred to AP.)

4

You're right, this usually happens when the DHCP client on the AP has another IP already assigned on the same bridge.

If the bridge has a static IP and a DHCP client at the same time, it causes a conflict. The DHCP server sends a NAK, and the client keeps renewing endlessly.

Just remove the static IP from the bridge or disable the DHCP client. Keep only one of them active and the log spam will stop.

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5

Thank you for the reply.

I reviewed again the DHCP-Client config in AP and noticed that I actually had TWO DHCP Clients in AP !

One client was on bridge and the other on ether1.

But the record for ether1 was grayed-out and there was a capital letter “I” so I thought that the record was disabled, however today I found out that it was actually enabled!

So I disabled the DHCP Client for ether1 and then the DHCP messages in the log of Router stopped.

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