DHCP Relay Issue with Pseudo-bridge

RouterOS Wireless Networking
1

I am trying to get DHCP relaying to work in a routed configuration. This is a switch from our previously all bridged network, and I am having a problem with DHCP not offering addresses to customer routers. Here is a rough approximation of our configuration:


DHCP Server --------- (eth1) MTK AP (wlan3) -------------- (wlan1) MKT CPE (eth1) -------- Customer Router/PC


The MTK AP is configured as a router and also has DHCP relay setup to relay DHCP requests to our DHCP server.
The MTK CPE is configured in Station-pseudobridge mode with wlan1 and eth1 bridged together.

This is what happens:

Customer router requests an IP address with DHCP discover packet.
MKT CPE forwards that packet through the bridge, changing the source MAC address to the MAC address of the CPE.
MTK AP forwards that request to the DHCP server.

DHCP server offers an IP address out of the appropriate subnet and forwards that as a directed packet to the MTK AP.
MKT AP forwards that request to the MAC address of the Router.
MKT CPE does not pickup the request as it is not addressed to its MAC address of the antenna.

As such, the router never gets the offered IP address. Is there anyway around this while staying in pseudobridge mode? I like pseudobridge mode because it allows me to setup queuing by MAC address of the antenna without having to know the MAC address of any of my customer equipment.

2

What a nice explanation you have provided! :slight_smile:

I see the same issue. In log files, it looks like an endless loop: DISCOVER, OFFER, DISCOVER,OFFER,… because the OFFER never reaches the endpoint.

I just set up the MTCPE to act as a DHCP relay and now the dhcp service works.

3

Unfortunately, about 1/2 of our CPEs are not Mikrotiks, so until then, is there any kind of work around on the AP?

4

We’ve used various different pseudo-bridge CPEs, mostly plug-and-play. They have all been able to let DHCP traffic flow through in a workable way. That includes when we had a main MT DHCP server relaying through a MT AP, as you have. I do not know how the CPEs managed to let the DHCP through, but they did. The only pseudo-bridge CPE where I’ve seen a problem passing DHCP is the MT.

5

If you’re using the typical ISC dhcp server, set “always-broadcast true” for the address class you’re having trouble with, in dhcpd.conf. The response packet will then traverse the CPE wireless interface.