Cisco DHCP Relay Troubleshooting (ip helper-address)

In many enterprise networks, DHCP servers are located in a centralized server VLAN while clients exist in many different subnets. Since DHCP relies on broadcast traffic and routers do not forward broadcasts by default, Cisco devices use the ip helper-address command to relay DHCP requests across networks.

When DHCP relay fails, clients cannot obtain IP addresses even though the DHCP server itself may be functioning properly.

Step 1: Verify Client VLAN Interface

The DHCP relay configuration normally resides on the Layer 3 interface of the client VLAN.

show running-config interface vlan 10

Example working configuration:

interface Vlan10
 ip address 10.10.10.1 255.255.255.0
 ip helper-address 10.10.50.20
 no shutdown

The helper address should point to the DHCP server IP.

Step 2: Verify Interface Status

If the VLAN interface is down, the relay will not function.

show ip interface brief

Example:

Interface        IP-Address      Status     Protocol
Vlan10           10.10.10.1      up         up

Step 3: Verify DHCP Server Reachability

The relay agent must be able to reach the DHCP server.

ping 10.10.50.20

If the router cannot reach the server, DHCP requests will never be forwarded.

Step 4: Check Routing Between VLAN and DHCP Server

Ensure routing exists between the client VLAN and the DHCP server network.

show ip route

If the route is missing, add or correct the routing configuration.

Step 5: Verify DHCP Traffic

You can confirm DHCP relay operation using debugging.

debug ip dhcp server packet

Look for messages indicating that DHCP DISCOVER packets are being forwarded to the configured helper address.

Step 6: Confirm ACL or Firewall Rules

Access control lists or firewalls may block DHCP traffic between the relay device and the server.

Verify ACL configuration:

show access-lists

DHCP uses the following ports:

• UDP 67 (DHCP server)
• UDP 68 (DHCP client)

Useful Verification Commands

show running-config interface vlan 10
show ip interface brief
show ip route
ping DHCP_SERVER_IP
debug ip dhcp server packet

Final Thoughts

Cisco DHCP relay problems usually stem from missing helper-address configuration, routing issues, or blocked UDP traffic. By verifying the VLAN interface, confirming server reachability, and monitoring DHCP packets, administrators can quickly isolate where the relay process is failing.