Hi guys,
I’ve a strange problem with obtaining an IP on a DHCP client through a trunked VLAN from an cable modem on an VLAN access port of another MikroTik.
There are two VLANs: 99 for my LAN and 300 for the connection to my cable modem. Unfortunately my cable modem is not in the same room as my router. But there is a MikroTik CRS next to it, which is connected to my Router via SFP. So I created an access port for a new VLAN 300 on my MikroTik CRS and trunked it through the SFP port to my router, where I created a DHCP client on this VLAN 300. Now it gets confusing to me: If the client asked for an IP the cable modem answers and the client gets an IP (192.168.100.10) with a very short lease time. This lease time changes multiple times before it expires.. and when it expires, it starts to count up to infinity. I don’t know how my cable provider provides IPs to customers, but this first IP looks like a temporary IP before the client should get the right one. If I connect a PC directly to the cable modem I see the same IP from the 192.168.100.0 subnet and after the lease expired, it gets immediately the public IP.. so it works and the problem is somehow related to my MikroTik VLAN config.
The same setup with the cable modem on VLAN access port on a HP switch, trunked to another HP switch and then connected over another access port to a WAN interface of a pfSense router works perfect out of the box. This also calls for a problem with MikroTik VLAN in this case.
Here you can see the DHCP client and the traffic on the vlan300 bridge on my router: https://www.youtube.com/watch?v=JOQHPpps-iI
I hope somebody could point me in the right direction.
Best regards,
Karsten
EDIT:
For troubleshooting I disabled the DHCP client on the router, created a second VLAN 300 access port on the MikroTik CRS and connected my Notebook to it. My notebook gets the 192.168.100.10, then the lease expires and there is no further IP. So the problem is not the VLAN trunk, not the router (firewall etc.).
If I connect the notebook directly it works, if there are two VLAN access ports on the same MikroTik CRS and the DHCP request/offer has to go through the VLAN, it doesn’t work.