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hugothebas
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Joined: Mon Feb 15, 2021 2:13 am

3rd party Layer 3 Switch + DHCP Server on mikrotik

Mon Feb 15, 2021 2:33 am

Hello,
I have an old Enterasys switch that has L3 capabilities, I'm using it to route internal traffic and send just Internet requests to my little RB750GL.
On switch side is everything working fine, I can access all networks that are on different VLANS and acces the Internet through Mikrotik.

I have 3 vlans on that switch:
VLAN 100 - lab_1 (192.168.100.1/24)
VLAN 200 - lab_2 (192.168.200.1/24)
VLAN 7 - transit (switch side: 192.168.110.2/30 - mikrotik side 192.168.110.1/30)

All these 3 vlans are tagged at switch side, although I think I really need only the transit VLAN tagged.

I have set the ip helper-address of both VLANs to 192.168.110.1 and I can see bootp packets forwarded by switch coming from the relative network at torch.

I have created both networks on dhcpserver and have created a dhcpserver running on relative vlan interface (vlan 7).
The problem is that mikrotik seems to ignore bootp packets and never gives an ip address.

Can somebody give me a clue on what I'm doing wrong?

Thanks!
 
sindy
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Re: 3rd party Layer 3 Switch + DHCP Server on mikrotik

Mon Feb 15, 2021 8:42 pm

All these 3 vlans are tagged at switch side, although I think I really need only the transit VLAN tagged.
It depends on how you have set up the routing on the L3 switch. If routing between 192.168.100.0/24 and 192.168.200.0/24 is provided by the L3 switch itself, it is enough to have the VLAN 7 / 192.168.110.0/30 on the link between the Enterasys and the Mikrotik, so it can be tagless there. If you have more than one VLAN on a link, at most one of them may be tagless there.

However, as far as I know, the RouterOS DHCP server currently cannot choose an address pool depending on the Relay Agent Information option found in the DHCPDISCOVER, so you cannot assign addresses to clients depending on the VLAN they are members of.

So whereas I think that setting the bootp-support property of the server to dynamic will resolve your original issue if the hosts really send BOOTP requests rather then DHCP ones, you will run into this next one immediately. So I'd recommend to tag two (or all three) VLANs on the interconnection link, create corresponding VLAN interfaces on the Mikrotik, attach a DHCP server dedicated to each subnet to the respective VLAN interface, and disable the DHCP helpers on the Enterasys.
 
mcasanas
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Joined: Tue Nov 29, 2022 12:29 am

Re: 3rd party Layer 3 Switch + DHCP Server on mikrotik

Fri Dec 02, 2022 4:10 pm

Could you solve your issue? I had some similar problem with PXE booting and DHCP snooping in a Cisco switch. Disabling DHCP snooping did the trick.

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