What can go wrong when setting up the DHCP relay server?

Zyxel_CSO

If a PC client connected to a Zyxel XGS4600 switch cannot receive a DHCP IP, you can follow these steps to troubleshoot the issue:

Topology:

+-------------------------+

|      DHCP Server        |

|    IP: 192.168.1.62     |

|  Connected to Switch    |

|       Port 28           |

+-----------+-------------+

            |

            | Port 28

            |

+-----------+-------------+

|           Switch  (DHCP relay role)     |

|                         |

| VLAN 1 IP: 192.168.1.254|

| VLAN22 IP: 192.168.22.253|

+-----------+-------------+

            |

            | Port (e.g., 10)

            |

+-----------+-------------+

|         Client          |

|  Receives IP from DHCP  |

|   Pool: 192.168.22.0/24 |

+-------------------------+

1. Verify VLAN Interface IP

Ensure that the VLAN interface IP is correctly assigned on the switch. For example, if using VLAN 22, confirm the configuration under the VLAN settings and assign the appropriate IP address for this interface (e.g., 192.168.22.253).

Since the switch will use the VLAN22 IP address to send DHCP discovery to the DHCP server.

2. Analyze Packets on DHCP Server

Use a packet analyzer tool (e.g., Wireshark) on the DHCP server to check whether the switch forwards DHCP discovery messages to the server. This can help identify if the server is receiving the requests.

3. Check Communication Between Gateway and Switch

Ensure the gateway can communicate with the VLAN IP (e.g., 192.168.22.253). For instance:

• Verify that the default gateway of the DHCP server can route to switch VLAN22 interface.
• Ping the gateway from the VLAN IP or the switch management IP to confirm connectivity.

4. Verify DHCP Server Authorization (If Using Windows server)

Check in the DHCP management console if the server shows as "unauthorized" (red arrow icon). If it is unauthorized:

• Right-click the DHCP server in the console and choose Authorize.

6. Test the Configuration

After making the necessary adjustments, test again if the client receives a DHCP IP. You should now see the PC client successfully getting an address from the DHCP server.

Following these steps resolved the issue in this particular case, and the PC client began receiving IPs through DHCP. Always ensure your DHCP server and network configurations align for proper communication.

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