DHCP Issue across switches

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DHCP Issue across switches

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DHCP Issue across switches
DHCP Issue across switches
2023-12-10 22:50:52
Model: SG2210MP   TL-SX3008F   TL-SG3452XP  
Hardware Version:
Firmware Version:

My network has 3 VLANs:

 

  • VLAN 10 (192.168.10.0/24): Servers/Management Network - Omada's Mgmt IP addresses/interfaces are on this network along with the Windows DHCP Server (192.168.10.12)
  • VLAN 11 (192.168.11.0/24): Clients Network
  • VLAN 13 (192.168.13.0/24): IoT/Guests Network

 

I have 3 switches in my network, connected in series:

 

  • Switch 1: TL-SG3452XP, my core switch which connects directly to the DHCP Server on VLAN 10 and on which I have also set up VLAN Interfaces (switch has been assigned the 192.168.X.1 IP address on each of the 3 VLAN, plus DHCP helper IP address of 192.168.10.12)
  • Switch 2: TL-SX3008F, on which both Switch 1 and Switch 3 are connected, via trunk ports (VLAN 10: untagged/native, VLAN 11 & 13: tagged)
  • Switch 3: TL-SG2210MP, connected to Switch 2 via a trunk port (VLAN 10: untagged/native, VLAN 11 & 13: tagged).

 

The issue is, if I connect a client on a port on Switch 3 (e.g. VLAN 11 or 13 untagged), the client cannot obtain a DHCP address. While if I connect the same client on Switch 1, the client will obtain an IP address. It seems that the DHCP broadcast packets do not reach Switch 1 from Switch 3.

 

If I assign the client a static IP while it is plugged on Switch 3, then it works normally. This means that Switch 3 only fails to forward DHCP/Broadcast Packets to Switch 1.

If I set up Switch's 3 VLAN interfaces with IP addresses and DHCP relay, then the Client receives a DHCP address. However this is not the expected behavior. Since there's a trunk port between Switch 1 > Switch 2 > Switch 3, Switch 2 & 3 should act as "dump" L2 switches and just forward all trafic to Switch 1 which should be assigning DHCP addresses via the DHCP relay configured on it.

 

My expectation is that Switch 2 & 3 would act as dumb extensions of Switch 1, which means that the behavior for all clients connected to Switch 2 & 3 would be the same as if they were connected to Switch 1. This is not the case and it's the first time I am facing this issue among different vendors. Am I doing something wrong? Is this a bug?

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Re:DHCP Issue across switches
2023-12-10 23:44:16

  @xrioann 

 

Check PVIDs on all ports. Make sure they are configured with the ID of the VLAN the port is untagged in. Also, restart the switches to clear their dynamic tables. 

Kris K
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